I hadn't thought of that. It's possible they're giving birth to actual demons to brink about a XK-Class-End-Of-The-World-Scenario. I still think they're trying to abort this one before it can be born again, each and every day.
As for why they're using those specific D class personnel (that is, people convicted of things like rape) is because the process is stated to be incredibly traumatic for all involved, because of it's horrifying nature. Indeed, the fact that they need convicted rapists to carry out the procedure is probably because it does involved something like abortion, carried out in the most horrific, traumatic way possible. They're the sort of people most likely to be able to make it through the procedure without blacking out halfway or something.
e- also, I believe the prior abortion failures were because they let it get too large, it passed a certain stage after which abortion is impossible and at least one was the result of a failed procedure, likely the failure to terminate the fetus before it got out. I think that if the fetus is actually born or close to being born - that is, that it manages to actually leave SCP-231 - then an immediate ██████████ event occurs.