This is interesting, but alternate take: maybe it's that Sensei has impregnated Maya (which is kind of implied in Baby Finches and the fact that Maya can keep reaching the roof) and hasn't Ami? There's that Ami lust event right after the 3rd reset where Ami really really really wants Sensei to cum inside her and he does not, and Happy Ami during fourth reset talks about how much she wants to get to the roof. I think Ami, or some force that is using Ami (since Lamb Legs implies heavily that there is a "real" Ami who is not a demon-possessed cum gremlin), really wants Sensei to knock her up so she can get to the roof during a reset, for some reason. I think this happening might be what sets off the dark route in the future (and no, I have no idea how that would work for the people not fucking Ami -- maybe she rapes Sensei the way Makoto does in White Oak Door?).
My theory of getting a girl pregnant in this game is that, something special/dramatic/traumatic has to happen along side with the lust event. For example, the event of Ayane which she got busted by Kirin, and of course Makoto's White Oak Door. I don't think simply having lust events where Sensei finishes inside give girls a chance to be on the roof, or else girl like Chika or Ami should've had their fair share of chance now (not to mention all the other milfs). Makoto seems to be the next in line roof girl not because of having a lust event after we start to know (or we think that we know) how this world works, but because White Oak Door is so intrinsically different.
(but the obvious hole here is that White Oak Door is missable, so this lust event might not be the determining factor)
Back to Ami, that Ami lust event where Sensei suddenly pulls out also seems to hint a different level of significance in impregnating Ami. There are other Ami's scenes where Sensei just finishes inside of her without consideration, so that abrupt pulling out is really standing out. Combining with the facts that:
(1) Happy Ami during the 4th reset seems to really want Sensei to knock her up so she can get to the roof
(2) In the revamp event Am I Awake, the nightmare sequence revolves around breeding your niece, the Ami with the creepy red eyes
(3) In Lamb Legs, it's implied that there is a real Ami other than the creampie-craving one.
(4) you can actually choose not fucking Ami
Thus, I can't help but feel like only bad consequences (like "otherworldly" bad; it's not like people need reminders that making your teenage niece pregnant is bad lol) await if Ami gets pregnant; or rather, impregnating Ami that is being used or possessed by some higher power will in fact be the end of Sensei, for the nightmarish Am I Awake becomes reality because the higher power now has had its way? Is a scene like Am I Awake exactly how some Sensei iterations met their fate?
(also, imagine a scene that Ami finally gets pregnant and gets to the roof, but not because she wants to make it through a reset, but to deal with the survivors, who make it to the roof through a method she greatly disapproves)
If that's true, for Sensei's journey to progress, I think Ami will simply not be pregnant no matter how, either because of Sensei's pulling out all of a sudden, or simply choosing not fucking her (or Ami can, like you said, rape Sensei the way Makoto did to have the plot point at the most mess up direction to thicken)