First of all, sorry to hear what you’re going through.
In regards to natural burial, my understanding is that it’s more legal than not, just that there’s regulations depending on where you live. It’s very rare that embalming is required by law. Burial vaults are typically required when embalming fluids are used to slow the spread of the fluids to waterways. Neither of these are considered part of a natural burial.
There’s a lot smoke and mirrors in the funeral industry that has led to wide misconceptions and outright misinformation. I asked if you were sure because the points you made are generally what a traditional burial funeral home would tell a client to steer them more towards their products. It’s awful that it’s become common for funeral homes to prey on those that are grieving. Absolutely despicable.
I hope y’all find a way that honors your grandpa without causing additional stress.