Trying to power 100 Arduinos from single power line
If your arduino is doing little more than reading buttons and comnmuniction on the RS485 bus I wouldn't worry too much about the power. At 10mA per Arduino the total consumption would be ~ 1A. Feeding at both ends would half the load. You could even feed in a couple of places in the middle.

Using buck converters would greatly reduce the currents (and the bills), so that would be a good idea if the current would be too high.

RS485 is a safe choice, but do you really need it? I think I would go for a daisy chain, where each Arduino (-nano) sends (UART output) to the next (UART input), modifying the message by insterting its own button data. With 25/100 = 0.25m between adajcent Arduino's this should be pretty reliable. 

A nice extra is that the ground potential is only an issue between two adjacent Arduino's.

(I once designed such a system, which did include enumeration and bootloading. IIRC the final configuration used 30 chains of 100 nodes each.)