Really? Again with this? DD got hurt every other day at that age. It sounds like your son is accident prone, as was my DD. It's normal, and it would STILL happen in your care, right under your nose, and it most definitely will happen if you move your child somewhere else.
I understand being worried, but your expectations are completely unrealistic and just because it's "Montessori" doesn't mean the kids aren't going to get hurt & have accidents.
At this point, I think you're not ready to have your child in a daycare setting, since I don't think you'll be happy with ANY care that receives other than your own. You are looking for everything wrong with this place, and I'm sure you'll get what you want.
I guess I'm not understanding why keep posting about this place... it's clear it doesn't meet your standards. You have 2 options - pull him out & keep him at home, or change centers, with the expectation that your child is at a clumsy age & will probably still get hurt from time to time.
IMO, he doesn't need to be in that setting at his age, and I'm not sure what benefit using a Montessori has at that age. He just needs to learn basics at this age, such as sharing, waiting your turn, interacting with others, etc. You can accomplish the same thing with play dates/ play groups, mommy & me classes, going to the park, etc., and just interacting at home, and not have to worry about him getting hurt or spending money unnecessarily.
Technically, in your state, the ratio for 18 month olds is 1 teacher to every 9 children. Also, I don't consider the biting incidents as an "accident", it's a normal part of behavior for kids that age & is to be expected. There has yet to be an "accident" where your son has been gravely injured.
ETA - to the poster after me that called me "harsh", it's called being honest. I have been there & done that with daycare & preschool settings & in my own experiences, there is nothing unusual or suspect going on here. Sorry if my matter of fact tone & honesty doesn't fit in your unicorns & rainbows world. Geez.