Probably some underpaid employee comes every week to refill the machines

Whether it's coin or cash I guess it will depend on which ATM and which bank you're using

No you can't get instant healing currently since that machine was broken and we are waiting for the IT department to send a technician

Since the adventurers guild is a bunch of communist losers the prices are legally fixed by the government after countless strikes and demonstrations

Every few levels there's an old wizard running a kiosk where you can get stuff cheaper

No reverse vending machine, the government deemed it illegal under monopoly/antitrust laws as whoever gets to control the selling machines can control the entire dungeon economy, but you can some times sell to a wizards kiosk and if you found coins or money you can deposit them at an ATM

The dungeons are still dangerous so nobody wants to live there but I would imagine some small communities or commercial centres and motels in some safe levels

And yes anyone can use a vending machine if they can pay this is capitalism after all