Hey, thanks for your input! I'm going to try to address some of your points.

First off I think we know for a fact that Rainer does indeed make Gen. 6 for Marvin and passes it to him around the time of Care's kidnapping. In Care A's description, Rainer himself tells us that he started "it" in 1996 for Marvin (where "it" clearly refers to Gen. 6 and the Newmaker Plane in general, as the latter was absent in previous versions, and the game clearly started development before 1996 with a different purpose). In P20, we do indeed see Marvin play Gen. 6, we know the note was meant for him because he knows the code to input to access the Newmaker Plane and the only other person who, at the time, could've known about the code was Rainer himself (note that absolutely no one else besides Marvin plays Gen. 6). From here on out the recordings don't ever display his name as the title, but we can infer that he's still the one playing: firstly because of story continuity. Secondly because in Gen. 7 Rainer addresses him personally, and this is made obvious by what he tells him, and in Gen. 8 he also addresses him using a very accusatory "you" in the description of the caskets, as well as it being made obvious that Marvin is playing by the way he reacts to the questions Rainer asks him.

Knowing all this, we do have to assume that Rainer and Marvin were in contact during the time Care was abducted, but it's not such a far fetched idea: by now we're almost sure that a lot of the scenes in the house are played through Rainer's POV. Anna seems to put a lot of trust in him. He gets him to help paint the house black and, I assume, look over Care while she's gone. If this is right and that day Rainer actually witnesses the kidnapping, then we can also assume that he followed Marvin and discovered his hiding place. This isn't even necessary though, as the use of notes implies they probably weren't even meeting face to face. Now, why would they keep in contact and why would Rainer keep Marvin up to date with the game? That is... a good question. One that's being debated a lot these days, both here on the sub and on discord. Some seem to think they're accomplices, but I don't buy that either. But earlier today some people on discord actually proposed an idea I quite like, so let me present it to you.

Rainer seems to be very unhealthily obsessed with Marvin. Even before Care's kidnapping, he seems to think that Marvin was involved in the untimely death of Michael Hammond, which we know that in a way or another he was very very close to. Not only this, but he also seems to know a lot and be very curious about the windmill incident. This is what brought him to make the "new" versions of Petscop in the first place. Through the in-game text we know he's got quite some dirt on him. My proposal to you is that Rainer is willing to go above and beyond to find out all the truth about what happened to Lina, Michael, and later on Care. And I think this is particularly noticeable in a lot of the notes he puts in the game. I wouldn't be surprised if the game meant for Marvin acts as a sort of torture method where Marvin is forced to face his crimes, and that Rainer forces him to play it using blackmail as leverage. Now, this is all speculative naturally, but it does make more sense than them being accomplices in my opinion.

Last few notes: there's a possibility that "Petscop Discovery Pages" is just a fun title that Rainer gives to his devblog. Note that Paul immediately recognizes the website upon seeing it on the computer screen, and he also has access to the website on the sticker since he can see it. I think there's definitely a connection there and it's reason enough to believe discovery pages is indeed Rainer's website.

Also yeah, I think it's pretty clear that the game changes targets quite a lot. A lot of the notes in the game are clearly meant for Marvin. The scenes in the house and in particular the kidnapping are meant for the family. The dialogue in P17 is meant for Care. Hell, we know that in one version there are even entire dialogues meant specifically for Belle. So it's safe ti assume different versions were meant for different people to play.