>>>Only ports from the same switch group may be master/slave ports
I know, but you not catch the point, ether2...5 on same switch group,
ether 2 linked on main switch with all other ether2, and the acces point can be put on any ether3..5.
If one of the RB750UP die, you must phisical move ethernet to other rb750...
>>>When this happens I want to ensure that the network will stay up.
The network can not stay up, but if your 3 ap cover the same area, and the other two are used for redundancy,
you can script one 750 for start another ap...
>>>Also, why would you consider it more secure to have one device?
Because if power from main line fail with overvoltage/discharge, you broken all the device at the same time....
Is more logical to reduce the fail point putting only two rb750UP,
and only two AP phisiscally connected,
leaving third ap and third rb750 as spare part, unplugged and unpowered.
About AP you must use one board like RB433AH, it have 3 ethernet, you must plug ether1 to first rb750up, ether2 to second rb750up and on rb433ah box power the board from both rb750up, not modifyng the board, but using dedicated connector
one script inside the two rb750up decide what device are powering the rb, or if hte connector are well made, both can power the rb, and if one rb750up fail, you not touch anything and all still working without the minimal interruption....
I'm Italian, not English. Sorry for my imperfect grammar.