The button is too small to drum on it with two fingers, I guess?

Depending how touch friendly the game is supposed to be, the mini-game could be different. There are minis that are ridiculusly easy on touch, but hard with mouse.  

And there is of course the itsy bitsy mobile phone displays. You can't see the screen with your hand in the way, tapping stuff. An outrage for a porn game ;-) 

You could cheat and connect a mouse to your Android phone. But I think it will not give more click events to the app. If you mush with many fingers, it tries to read a gesture or interprets it as a single tap. I tried with some app and that one registers only two clicks, when I alternate between tap and mouse click.

Thanks. I usually offend people by overanalyzing ;-)

But the perspective that the game is on mobile too is eye opening. Its different for something like visual novel. I remember playing one game on browser on mobile specifically because the mini game they had was far easier on touch screen, than on browser on pc.

I like lewd games that have the lewdness part of the game and not only just as a bonus. Like doing a match-3 game and a girl stripping for your highscore. I also like consistency. Only male zombies is inconsinstent with the reality, that male are only half of population. You could explain by women being immune,  women not transforming after death,   and so on. Personally I find the theory hilarous, that zombiism was introduced by  a viagra like drug that really only affects penis anatomy. But you could totally introduce female lesbian zombies at a later date. Having differnt types of enemies is the norm for what essentially is a tower defense game. You can also hand wave it by claiming that the female zombies do gather behind the siren, but do not bother entering the police station, because she does not look "tasty" to them. 

Your type of zombieism is obviously related to sex. They do not eat our heroine, they screw her - and do not infect her by doing so. If this is unique to her or  attributed to the cause of zombiism would be plot points. Your game has more ... lore than stuff like plants vs. zombies. You have story.But yeah, you can also have visual novel style base mechanics and plot and have a naughty zombie defense shoot out in the night. The whole shooting could be considered the mini game in that case. 

Of course you need not explain eveything to the players, especially if it might be spoilers. But to a nitpicker like me, it bothers me, if she can throw grenades while being  screwed to death (literally, she loses health), but not to prevent  that from happening. So I let my imagination run wild and find other ways to explain it. I came up with three explanations so far. First the zombie that has finished on her runs away and tramples all the others while doing so. But that does not explain why they wait. Second,  the other zombies go away on their own, because it is a lewdness zombiism, and just because they are dead, does not mean they are comfortably with touching balls with other male zombies. And third, some convoluted explanation why she only dares use that flashbang thingy while they are distracted from watching the show.  And if you want to go full on silly minigame, make it the wave of orgasm energy that staggers the other zombies ;-) (actually, now that i wrote that, it does not even sound silly anymore. zombies are super natural, so why not defeat or repel them with sexual energy)

Nighttime zombie attacks are a valid concept. It is the whole plot point of I am legend movie. They do not like sunlight there and only come out at night.

Regarding the siren, it its the police siren on the car. I do not even know if they have a loudness setting. I took it as, how extensive she drives patrol to "collect" zombies to walk after the police car. Or did she only drive around the first time and just shortly activates the siren the following nights, without driving. Fuel would be an issue, but still, any loudness you can compensate with time the siren is on, and ultimatley it is just: how many zombies do I lure to my base this night.

Of course genre savy zombie fighters would just use a siren on a prepared battle ground and stick it on a high pole where they cannot reach and when they are gathered,  shoot them from behind or destroy them by other means. But she is not genre savy, she is a rookie that repeats what worked the first night. But even here you could explain with the lewdness factor. They are not attracted to the siren, but to the attractive female driving the car. If manly man necrophobe officer would try to do that, it might not even work. She could maybe even take her top off to lure more zombies, if you want to play with that idea. (If you want it to be the reason and want her to find out and exploit it. She is a rookie after all.)