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Great questions! Some of the vagueness is intentional, while some of it is a result of the form factor and time constraints of making this for a game jam. Here are some suggestions:

• Looking at what the locket does, I imagine he loves the sphinx but never has that love reciprocated. I left this vague, though, thinking it could be a source of inspiration to continue the campaign after the gauntlet. Even then, I don't think Fang would ever tell the puny adventurers (i.e., dinner) his reasons for wanting the locket. It's more of a "You better get it, or else . . ." 
• I put the locket in the loft since (1) it needed to go somewhere and (2) Fang would have an easy time getting there. You could do this up a bit more: Make the entry to the loft trapped, and make it the manticore's den. I wish I'd thought of that when working on this during the game jam!
• The bandits' reason for following Fang: fear! He'll just eat them if they disobey. 
• The scarab is his main goal, and while he has no problem abandoning his men to his fate, he's got an ego. I bet he'd try to exact revenge, blame the PCs, etc., but if the tide starts to turn against him, he'd flee--and hold a grudge. 

When running this, I try to keep the pace fast, the action high, and the manticore terrifying, demanding, and a tad goofy. Meanwhile, the bandits are afraid of everything, including the monsters below and the potential retribution of their manticore lord. 

Hope that helps! Let me know if you have any other questions.