Hello, I'm Xan, EiC and co-developer for FFLE. Thank you very much for your feedback, and I have answers to each of your parts!
1. The 'Core Mechanics' page, which is the only place in the entire document where boons and banes are defined, is not linked in the document outline. I will admit, when I got to the section of the prelude about the history of various editions of the game I assumed it was not going to contain useful information, which is partially on me.
There is also the Glossary, which follows the Table of Contents and is before Chapter I, which has more information about various mechanics, such as Banes and Boons. We also have a robust amount of information on the SRD FFLE Wikidot!
2. On page 23: Advancement, it is claimed the table contains two columns for 'damage dice' with the second being for 'warrior class advancement'. There is only one column dealing with damage dice, and the table itself calls it 'bonus damage'
The Warrior dice column was something being considered and then removed at a later time. That sentence is an artifact from that time. Sorry for the confusion! However, if you would like to try using that advancement scheme for Warriors, it is listed as a beta item in the Character Creation - Advancement section of the FFLE Wikidot.
3. Companions seem to be free action economy that can be traded for free survivability as needed. I assume companion classes are balanced around this, but it is still concerning given the potential for someone to have one as a 2-point 'class feature' enchantment on an item.
Companions only have a free portion of the Action Economy if using their Minor Action, and it always occurs at the end of the Round; it cannot occur during any Participants' Turn. Additionally, in order for a Companion to absorb damage, it must be chosen as your Reaction to that damage, which you only get one choice of Reaction per instance of damage, meaning you can't choose to Dodge and then try to choose your Companion as the Reaction. Furthermore, the TN for the Reaction is equal to the damage that would be dealt; upon success, the damage is halved. Upon failure, the damage is negated entirely, but the Companion is then unavailable for the next entire Round, meaning you can't use its Major or Minor Action.
4. I assume 'Flurry' only multiplies your PCM bonus to damage, and you only take the rolled damage from one attack, but the wording is ambiguous - I initially read 'attacking the same target repeatedly[...]' as making multiple separate entire attack actions against the same target.
Flurry directly states "You may declare a Flurry as a basic Weapon Attack, attacking the same target repeatedly until you fail the Attack Roll or make a number of Attacks equal to your PCM." You are rolling multiple Weapon Attack Rolls to make multiple Weapon Attacks as a single Action, meaning you are rolling damage for each successful Weapon Attack Roll, and adding your PCM as additional damage on top of the normal damage calculated.
5. If 'Red Carpet''s permanent activation of a stage also bypasses the special activation requirements of foe down hoedown, adding some text to the effect of '[make a sustain] or meet any special activation conditions' could be wise.
Red Carpet's permanent activation does NOT bypass Foe Down Hoedown's requirement to first defeat an enemy before using the Stage. Remember that Specific beats Generic when looking at rules, and the Special requirement for Foe Down Hoedown is more specific than the description of Red Carpet.
6. 'Spell Flurry' has some similar ambiguity to flurry - I assume multiple hits just up-rank 1 spell and don't cast multiple spells. Other questions on 'Spell Flurry' and attendant features: are Innate Spells still capped at rank 6 when spell flurry would rank them up? if so, does pushing them past maximum rank cause an automatic critical success as it would for a spell that maxes out at rank 9, or is it simply wasted? What is the actual benefit of causing an automatic critical success with a spell attack that has hit anyway - the spellcasting chapter mentions nothing about e.g. extra damage on a critical success. Do all spell flurry attacks generate potential Extra Effort Dice?
Spell Flurry bypasses the Casting Roll, assuming that the Casting is a success. You are correct that multiple Spells are not being Cast; this is why Spell Flurry says it counts as one Cast for the purposes of other effects. Innate Spells are always capped at Rank 6; this means they cannot get the automatic Critical Success for exceeding Rank 9. Critical Successes with Spell Attacks are supposed to also double the damage dice like Critical Successes for Weapon Attacks; this is reflected in the Spellcasting section of the FFLE Wikidot. All Spell Flurry attacks can potentially generate Extra Effort Dice, as each Spell Flurry attack is a separate instance of damage or other effects.
7. While I think it is fine that many equipment feats are the potential effects of magic items, Open Binding feats should probably be forbidden for this purpose.
Magic Items are not something a player can design bespoke for themselves in a normal session; the GM would decide what Magic Items are found or Acquired through the Acquisition Roll, and what Enchantments the Magic Item in question would have. In this case, it would be up to the table and/or GM whether they want to include the Open Binding feats as valid for use with the Feat Training Enchantment.
8. At high levels, Armor Expertise II can force certain tiers of spell to explode when targeting the user. This is perhaps undesirable.
It wouldn't force exploding dice, since Armor Expertise II steps down the damage dice before they are rolled. It can increase the likelihood of Power Surge occurring since there are fewer die faces, but that doesn't force Power Surge to occur.
9. Absent certain synergies, it is a higher increase in expected damage to cast a spell at its lowest available tier and convert any bonuses to the casting roll to extra effort dice than to raise it a tier.
Extra Effort Dice are not subject to any Damage Modifications like Damage Vulnerability or Damage Reduction. This means that, for example, if casting Rank 1 Fire and getting a bunch of EED, only the 1d8 would be maximized from an opponent having DV 1 (Fire), and the EED would simply be equivalent to what you roll. There is a trade-off Casting this way.
10. As mentioned regarding 'spell flurry' it is unclear what the benefit of a spell critical actually is. The spell attack roll section could generally use more detail, not only on this but on other ways spells are similar to or differ from other attacks, e.g. is the character's MCM added as extra damage? Do they gain extra damage dice from leveling? (I assume not, but I also assume criticals are supposed to behave like attack rolls, and the lack of a feat or such to add the MCM as damage surprised me)
Your MCM is added to the totals for damage-dealing and healing Spells, and this is also reflected in the Spellcasting section of the FFLE Wikidot. Spells do not gain more dice as you level like Weapon Attacks do - the additional dice come from the higher Spell Ranks. Finally, as previously mentioned, Critical Success during a Spell Attack Roll is an automatic hit and doubles the dice of the Spell, but not the Extra Effort Dice.
11. Absent synergies or luck with exploding dice, it seems 'Break', which does scale with MCM, outperforms most other damaging spells in the hands of a primary spellcaster. Additionally, it would be worth clarifying if 'Break' still does it's damage if a target cannot be petrified.
Nowhere in Break's effect does it say that the target must be vulnerable to Petrify in order to deal damage. It deals damage regardless. If you could clarify why you thought something must also be susceptible Petrify in order for Break to deal damage, I would love to hear that input; it could possibly help us to make better effect descriptions in the future. In addition, Break's damage is not rolled. Since it is flat damage, it doesn't get mitigated or boosted by Damage Modifications such as DV X (Earth) or DR X (Earth), and Critical Success would not double its damage since Critical Successes double the amount of dice rolled, not the damage total.
12. For enchantments specifying an element, is "non-elemental" a legal pick?
By definition, non-elemental is not an element. If non-elemental could be picked for elements, it would defeat the purpose of non-elemental being a category.
13. For enchantments specifying conditions, I assume only those in the 'conditions' segment of the combat rules are valid, not any bespoke conditions from spells or class features, but it is perhaps worth putting this in writing. Likewise, given the unequal nature of negative conditions, 'status strike' could perhaps have a limited pool of conditions by tier, rather than treating Stop & Bleed as equal in value.
Correct, bespoke conditions not included in the Combat Chapter are not valid for the Enchantments which allow you to select Conditions. Additionally, determining whether the selection of Conditions is limited for Status Strike would be something up to the table or GM to determine. The philosophy behind FFLE is we give people a toolkit with many cool things, but it is up to them how they would like to use it.
14. Two NPC categories are listed as 'dragon'. I assume the second, with an icon resembling a person, is intended to be listed as 'humanoid'
Correct, that second "dragon" category in the 1.2 PDF is supposed to be humanoid. It is an editing error from 1.1, which still has it listed as humanoid. It is also still listed as humanoid in the FFLE Wikidot.
No valid and constructive feedback is disheartening; I enjoy having these conversations and continuing to improve our game.