Thanks for the feedback, it's always helpful hearing both what people like and DON'T (as long as they're polite) because it's easy to be focused on fixing one set of things when people actually care about others.
I'll just start off by saying the combat, while fun, is definitely very primitive in this first chapter. And while I'm never going to make something super in-depth, I am continuing to tweak and improve it as I work on chapter 2. Once that's finished and playtested somewhat, I may back-port the relevant improvements to 1. But let's get to specifics on what you're looking at:
The staircase fight IS really hard. Part of this is on purpose: it's supposed to frustrate you into giving up and using the sex option. But there's a sweet spot of frustrating, but doable, and I may end up moving down just a hair in difficulty to better hit that zone. I don't think it will require any new skills, just playing with the number of enemies, how fast they deploy, and some other stats to get the balance right. I've adapted/changed the way energy drinks work in chapter 2 provisionally: now instead of being the way you recharge, your energy re-ups after every battle, and the drinks instead act as a "supercharge", maxing your energy for 3 turns so you can spam the spin attack for longer (but not forever). I'm balancing for this by making the drinks much rarer as a pickup, but I think this usage is a lot more fun to play. This alone would probably go a long way to making that fight more doable, though it would still have to be balanced.
Lust is admittedly a half-finished concept. You're right, there should be a way to drain it off after battle. On the other hand it isn't THAT big a deal, as you have practically infinite heals outside of battle thanks to Patch Up, and I like the idea of it being something you CAN'T control, an obstacle that kneecaps you more and more as you sink deeper into corruption. Right now though it's just kinda there though. In Chapter 2 I'll try to either cut it (there's another mechanic coming in) or flesh it out more. And again, based on what I end up learning there, I may eventually port some changes back to chapter 1.
I'm pretty happy with Patch Up. The idea is that items are for healing in combat, and Patch Up is for healing outside of it. This means you never have to worry about being low health IN GENERAL, just during the course of a fight, and as a dev that makes the possible scenarios I have to consider a lot smaller, and makes item pacing etc easier.
Lust combat, or battlesex, isn't so much about the mechanic as it is using a mechanic for storytelling. That said, resisting (or not) IS the mechanic. First that gameplay doesn't happen at all, then it's easy to escape, then it becomes harder and harder to escape until it just happens with no option to avoid it. As these kind of RPGs go that's kind of par for the course: the enemies do things that make you more and more vulnerable, and then pounce, and you have to try to struggle out of it. If you had ideas on how that big could be more engaging though, let me know. Always open to pondering possibilities as long as they're not too wild.
Again, glad you're enjoying it, and thanks for the feedback!