It's perhaps not impossible to have an achievement system, but is it really necessary ?
There's already the event tracker. If one value is still at 0, then you missed something. And if really you're stuck, you can unhide the names of the trackers, which will help you find what you can have missed ; there's really few that don't have a pretty explicit name.
By example, what is probably the more forgot event is the one name "(barmaid)" (I'm still waiting for her to have a name
). Once you know that, it's relatively easy to know what to do.
But, without guaranties, what I can perhaps do is to add something telling you that you've reached the higher possible value for this event. There's some, like the one with the barmaid, that need many interaction between each step, so knowing that you don't need to flood the action anymore can be interesting.
Once again it's a bug that come from the game itself. There's two/three like this, and alas I can do nothing to fix them.
Technically it's possible to correct that, but it imply that the mod will no longer be compatible with future updates. Therefore, I prefer to keep the bug unfixed, it's a small price to pay to keep one of the most interesting feature of this mod: its compatibility with what isn't added yet.
Well, in fact it's more the opposite
An achievement system need to have a view over the game (have the player passed through this part, have this variable more than this value, and things like that). It's not necessarily something easy to do, but still way easier than a gallery that would replay the scenes. For this, you need to have control over the game flow and ensure that the variables are neutralized.
By example, for Megan to reach her last level of corruption, you need to go to the Spa with Jenny and Nicole until your see Megan's husband, then you'll need to go to the playground during night. And obviously there's a variable that take count of this. If you replay one of those scenes, boom the value of the variable will change and possibly break the game.
It's already relatively dangerous that I offer the possibility to lower the level of corruption and so to replay this kind of scenes. It's clear that I'll never go further, the risk would be too high.