We had Lexie spayed about five months back and found immediately that she would not wear the cone of shame, no chance. It was the smallest they had, and there was no way of fastening it tight enough that she couldn't slip out. Hers were also internal stitches, and to be honest, I only caught her licking and nibbling at them once, after about four days. She sleeps on top of me at night, and woke me up jiggling around trying to give them a good chew. I told her no and fussed her until she fell asleep again, but I can only imagine the healing wound must have been starting to itch a bit.
The vet was very pleased with her at her check up and when I explained that we had abandoned the cone almost immediately, she commented that getting them into the cone, having them wriggle out and then trying to get the cone back on was so stressful in itself that they probably aren't worth the bother most of the time.
I can't even imagine how I would have gotten a baby grow on her
I can only think that you'd have to cut the bottom out anyway for them to use the litter tray, and I'm fairly certain Lexie would have found a way to wriggle out of it.
It's difficult since your vets are insisting on it. My advice would normally be to just keep an eye on her and the stitches, and ensure they're not inflamed, oozing or beginning to smell, but I know if I'd taken Lex back to the vets because of any of these things, they would have just dealt with it and looked after her without guilt-tripping me for not persisting with the cone.