so the twist of this one is that one of the colleagues is an undead superhuman who managed to survive however she thinks she killed them, then survive being buried alive, then rip themselves out of the earth so they could report it to the police and get revenge?
a bit more of a stretch than the others
It's hard for a slightly built woman to kill four people with a shovel, so she didn't manage to do it good enough (of course there's a possibility that she used another weapon, it's still difficult). For her to bury four people deep enough would take days, so she probably only shoveled enough dirt over them to just cover the bodies, and just bet on it to take a while for someone to discover them (no luck). The reason for the shovel could be that they were out there to dig up the loot, otherwise it would have been difficult for her to get the others there without trouble.
It's very possible to write around the problems and make something like this into a good story, because someone did.
This is very similar to the plot in John Dickson Carr's 1930s crime classic "The Hollow Man":
Three French-Hungarian brothers commit a bank robbery, hides the loot but are caught and imprisoned in a plague-ravaged prison. They fake sickness, pay a guard and are put in coffins and buried in shallow graves outside the prison. The strongest brother have a small tool so he can dig himself out and then his brothers. He digs himself out, but instead of helping his brothers he escapes with all the money himself. In the morning the open grave is discovered and the two others are dug up. One brother is dead, but the other is still alive and is locked up to serve out his term. After many years he is released and sets out to find his brother who is now very wealthy and living in another country. And that's when the fun starts!
I would advise anyone to find and read this book, it has been selected as the best locked room mystery of all time and is also high up on lists of the best crime novels ever written.
My own view is that it's not really literature, but (like Carr's other books) more an entertaining intellectual puzzle. In this case that is reinforced by the "detective" Dr. Fell breaking the fourth wall by admitting that they are in a novel, and discussing possible crime novel solutions. I love things like that so much that I sometimes write way too long about it.