About the theory that Gillian is Mike’s girlfriend: Isn’t it too unlikely that neither Ian nor Mike have any idea who the other one is?
The timeline in itself fits decently. I vaguely remember Ian mentioning early in the game that it’s almost a year since he and Gillian split up, and also Mike mentions that he’s been together with his girlfriend for about a year. Apart from that we also know that Gillian's breast size fits with how Mike describes his girlfriend.
But if your girlfriend doesn’t just cheat on you as a one night stand, but actually meets someone else who they end up dumping you for; wouldn't you at the very least have checked their social media to see who the person is? And if you're the other guy, wouldn't you do the same to see who the person whose girlfriend you stole is? After Ivy's birthday party it seems clear that they haven't seen each other before.
If it still turns out to be true, I agree that it’s a bit lazy. Some of the earlier coincidences worked really well: Like Jeremy turning out to be both Ian's friend and Louise's "boyfriend" (it's introduced early in the story when all the connections are being established), and Cindy's photographer who's also Lena's ex (we're aware of it from the beginning even if the characters aren't, and as a hotshot photographer it makes sense that Axel has a lot of connections). The revelation about Cherry and Axel felt a bit more far-fetched—like something that's forced into the story to make things add up. Gillian being Mike's girlfriend would be in the same category for me. That's both because of the total amount of coincidences and the fact that those coincidences are more eye-catching later on in the story when the universe is established and we assume we know all the connections between the characters.
Still I wouldn't consider it a big issue if we're being pragmatic. From a narrative point of view there's a lot you can do with that connection, and with how Lena's cheating with Mike mirrors Gillian's cheating and Axel's cheating. And for the moral dilemma of Lena's affair with Mike to be as effectful as possible, I think his girlfriend should be introduced at some point—if it actually turns out to be Gillian, Eva won't have to introduce yet another character for it to happen.
Either way the prologue confirms that Eva has changed her mind from her earlier comments about Gillian's role in the game. I assume that means she has some clear plans for what she wants to add to the story. Regardless of whether Gillian is Mike's girlfriend or not, I think this is a good moment to do that: A surprisingly big part of chapter 10 dealt with Ian's and Lena's feelings about Lena's possible sexual involvement with other guys. Gillian's return is a very natural ingredient in that mix.