No longer possible is incorrect. The DTV is a brand new visa. Either it hasn't been possible to open a bank account with this visa so far, or they don't know what it is and don't want to open an account for you. Try another bank/branch or seek out an agent.
Strictly speaking no, but it depends on how often or the nature of this volunteering. If it's a regular thing, whereby she would be associated with a charity then she would need to be on the correct visa class. If it's a one time thing alongside lots of other people, like tourists do sometimes, just handing out a few gift baskets to some poor hill tribe villagers for an hour that should be OK.
It's too early. Wait until maybe a week or two before the first 180 days are up, meaning early January and then, maybe, will we know.
In Thailand, never expect a lot of notice given for anything. That's not how they do things here. It's almost always a last minute thing.
We saw that during Covid, when tempers flared because Thailand was the last country in the region to announce an automatic extension of stay for all foreigners stuck in the Kingdom.
They should have made an announcement 1 month earlier, as they knew borders would be shut.