I work full time for a crematorium/mortuary care center. Speaking from experience (as far as U.S death care goes), it is extremely unlikely that you would be buried alive.
It's untrue that most bodies receive autopsy, but if someone is being buried with a service, it's very likely they will be embalmed. If by some slim chance that someone is still alive and nobody notices when pulling them onto a gurney, carrying them out of the house/hospital/etc., driving them, moving them into container/mortuary space, performing I.D checks, visual I.D check from family days later, (the list goes on-) AND someone isn't woken up by being cut into for autopsy- mmmmmmm you know what I'm trying to say haha.
Off the top of my head when I hear about "scratches on coffins" I think about places in the world where "someone died? okay check for a pulse, no? bury them. done."
Cremation too. I wouldn't worry.. I think the average turn around time for cremation is 2 weeks if there's no delay in paperwork/payment to the funeral home. From pick-up to going in the retort. The funeral home has to get permits approved along with payment/approval by next of kin. There's tons of times a body is visually checked for identification throughout the process.
It's a valid thought to be afraid of, but I honestly wouldn't worry about it being a reality. If you want to avoid all possibility, every tiny possible chance, just state in your will that you want to be embalmed.