You can do a lewd story without being explicit, if you want to. Zombies already steer any age recomendations toward mature. But I am here for adult games, so I do not complain ^^
A mini game for escape is a nice touch, but it needs careful thinking what you want it to achieve. If the mini game is reliable winable, the zombies pose no real threat anymore. The press a button quickly mechanic is often seen, but I do not like it for various reasons. Maybe others do like it for other reasons. (It is bad for mouse elbow/carpal tunnel, it breaks mouse buttons, it does not involve skill or planning and it is quite easily overcome with any enhanced hid: the poor man's way is to activate inbuilt windows keyboard mouse and hammer away on mouse button and keyboard).
And, ahem, the thing with the medkit for grenade would work. I would clear the room. It was not meant to finish level earlier (although I never paid attention. But I would have assumed that the zombies were in fact defeated, because you show some kind of grenade being used). The reason was to have more room to fire with unlimited pistol ammo and not get assaulted every 5 seconds. I did not suggest to remove the effect of grenade, but to consider how it was told. If you establish that she does have that equipment, one would ask, why does she not use it before, because it is super effective to avoid being assaulted in the first place. The other zombies could also just have wandered outside because their prey was already "in use", and return once their buddy is done. You do have the slight plothole, why zombies should wait , just because the living human is already ... engaged with another zombie. In all those zombie movies, they gang up on you. But if they , well, do not like to doubleteam, they might as well be repelled by a zombie already screwing. (Yes, I like consistency theory crafting like that. This behaviour would even play into the lewdness of the zombies, as "normal" zombies do eat you and do not mind other zombies eating at the same time. It would even allow for special zombies that like to gangbang. If one of them reaches her, all the other gb-z tag along, making the mini game harder.)
For English, a trick is to use translator page twice. Better yet, using two. You put your english in translator and look if it sounds funny after translation in your native language. If it does, try fixing it and putting it in the other translator back to English. But do not worry, it is fine detail and not important. English is good enough. Even finer detail would be that time is not used as a currency. It is like reading 5 money. It can be done for style, but most just use abstract "Energy" or action points, making it easier to incorporate stuff like energy drinks and drugs. Or depending on scale even hours/minutes directly, if you have fixed events depending on time of the day. Also, set the siren sounds funny. You set an alarm clock. But a siren you turn on. And what she actually does, is, lure zombies (with the siren by driving around with the police car. Lets hope she has enough fuel) (Disclaimer: English is not my native language either.)