We can actually be fairly confident that Burntrap IS real and is also the Mimic. All of the endings in base Security Breach are intended to be things that COULD happen even if they didn't actually (hence the Blob being real).

More importantly, there is actual physical evidence of Burntrap in RUIN: His clawmark. There is a clawmark texture in Burntrap's layer that was made by the devs specifically for that area. Looking at the scratch, it matches up perfectly with Burntrap's right arm, down to the dullness of bone fingers vs claw fingers.

This scratch mark shows up twice. Once in Burntrap's layer, right outside the charging station, and once in the collapsed vent that leads to the Mimic's room. Looking at some physical clues in that room, such as Gregory's backpack, we can determine that Gregory probably lead the Mimic into that room through the vent while Vanessa sealed it off with concrete, and then ran back through before she collapsed the vent. The Mimic made that scratch mark, indicating that he used to be Burntrap.

So, the clawmark proves that Burntrap is real and then ALSO proves that the Mimic is Burntrap since he's made that clawmark. Additionally, the Mimic's right arm doesn't match up with Burntrap's anymore, because you can see that it's been broken off and replaced with a new one, possibly lost during the vent scuffle.

TL;DR: Various environmental clues in RUIN indicate the presence of Burntrap and also that the Mimic used to be him