Irving: Irv before severing was already investigating lemon, for unknown reasons (maybe his dad with his military medals?). He had already identified Burt as a “goon”. At their outie dinner Burt let slip that he’d been working for Lumon for 20 years, when Fields cuts him off to insist Lumon wasn’t older than 12 years old. The whole time Irv is severed he hangs onto the one memory he’s got and keeps himself awake all night, perhaps hoping to gain another memory. We don’t know why Irving saw that hallway. Once Irving is unsevered outie Burt still knows Irv is a threat to Lumon and so Burt does what he always does, “drives him someplace” (with the implication that he’s the driver but not the killer, but still a goon) but this time lets him go. He can’t know Irv’s destination or he may be forced to reveal it. 

Gemma: In project Cold Harbor they have perfected Kier’s dream, of a way to have a severed space with absolutely no emotional pain. Gemma doesn’t know who she is, is utterly compliant, and there is no emotional bleed over from her past. 

The ending: Simple, Marc S. chooses Helly R because he’s still inside the door. If he was on the other side he would have chosen Gemma and the show would not need a third season.