Of course. Especially if you're Japanese. I've only ever seen citizens of countries like Sri Lanka be required to enter a country on a specific flight and in that case it was a flight from Bangkok to Phnom Penh.
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.