I'd disagree with you on this. While I'd agree - and it's supported by pathology - that Meredith would most likely not have been dead by the time the assailant(s) (WHOEVER he/she/they were), I think it would have been clear to anybody in the room that Meredith was extremely unlikely to recover after receiving those wounds - and most certainly not without dramatic medical intervention. She would almost certainly have quickly lapsed into unconsciousness (mercifully minimising her suffering), and would obviously have been bleeding profusely. So it was more a question of "if", rather than "when" she was going to die.
I also think it extremely unlikely that Guede would have wished Meredith to recover (even if in a fantasy scenario), if he were the sole assailant. Any recovery by Meredith would have resulted in Guede inevitably being jailed for a very long time for sexual assault and attempted murder. I'd think that Meredith's death would be a "preferred" option for Guede at that point.
HOWEVER, this adds an extra dimension into why Guede would have elected to lock the door, under my belief that it was done primarily to deter discovery for as long as possible. If Meredith was still alive when Guede left the room (as is likely), then Guede's locking of the door would serve an extra, important, purpose: if a flatmate had returned shortly after Guede left (which he had to assume was a possibility), then any discovery of Meredith - still alive - might have led to urgent medical intervention that might conceivably have saved her life. Guede would then be in big trouble - for obvious reasons.
In addition, had Meredith not survived, but had nonetheless been revived to the extent that she could whisper or write, she could have named Guede as her killer. Either way, Guede had to minimise the chances of Meredith being found, and locking the door was the easiest way to do that (short of moving her from the house, which clearly wasn't a possibility).