1. and 2. I think it won't be locked unless you've actually choose to break up with them. Sex here is driven by progressive factors so it is multifaceted. First of and foremost is your biological sex (Having a dick mostly gives you a top route, having a small dick or pussy would give you most bottom scenes) Respect/Disrespect range will tell you if you are able to have sex too. Being mid of those would hide all those scenes.
Again, Respect = Dominant
Disrespect=Submissive.
(Also a lot of dominant scenes, needs a penis, large ones)
(You can be a dominant, and have a pussy too. But it shows lesser sex scenes compare on having a dick, plus other character prefers dick over pussy, namely: Rylan and Phoenix, or not such as Skru'Vaks.)
Desire/Hate range - I never gotten be hated atleast (but the description of the game you can have sex even if the characters hate you.)
3.Idk but mission wise, There are missions that spans with time, repairing, waiting for laboratory results, I wish I could have knowledge more about it.
4. You have more grasp of the stats. There are choices that are mostly DND-ish such as appearance. The lower the numerical value you'll be frown upon as ugly. Magic stats on the otherhand, unless you've reach Harmonia and studies actual Magic skills. Magic stats is useless on the earlier story arcs. Literally. By the way, you can click the name of the stats in the game itself to see their described utility.