1. In regards to the endings yes there will absolutely be a hint system. The goofier ones will more or less tell you what to do but the more major ones will be vague enough that you'll still have to work out the finer details. Though currently I'm unsure if the hints should be unlocked upon achieving ANY ending or only after getting one of the main 5 endings (A-E).
2. As for the games price the only real answer I can currently say is, I don't know. It's something I seem to keep putting off coming to a decision on and likely wont until I firmly know just how long the game ends up being. $13 is a price I think about often simply because 13 is a fairly crucial number to the game as a whole but it's a bit of an odd price and I'm not sure it'll be appealing to people haha. I definitely don't see a world where the game ever exceeds $20 however. Somewhere around $10--15 is probably where it will land unless the average playtime comes out MUCH longer than I'm expecting.
3. When it comes to the costumes I always intended to either A) Reflect them in lewd scenes or B) make certain scenes locked to certain outfits. As my focus shifted however to the guaranteed release I felt it would be better to simply make one standard set for launch that everyone can view. And then post launch updates can be done to make the CG's reflect different outfits as well as add even more. I really appreciated the way Stellar Blade continued to give free costume updates after launch and would like to do something similar if I can manage it.