i can maybe offer an option for your last question
look on ebay for a "reversing contactor" they are more commonly 3 phase but don't let that concern you, you simply use 2 of the three contact sets for single phase
they are about bullet proof for reliability, and are failsafe when it comes to not interconnecting both sources.
they are basically two contactor/relay units, that are mounted on a common plate, they have a mechanical interlock and an electrical interlock on the coils (typically)
what the interlocks do is guarantee that both cannot be closed at the same time, only one can close and the other must remain open circuit.
used they are quite reasonable, just remember to check the coil voltage requirements, although the coils are generally replaceable why go the added expense when the proper voltage coils are offered on the units sold on ebay. typical voltages are 24vac, 120vac and 240vac
although it is possible to find other less common voltages as well.
also keep in mind that a small control transformer can provide the needed coil voltage if need be as long as you can provide either 120 or 240vac.
don't be too concerned about used contactors, because they generally have replaceable contacts and come in a kit to rebuild them. doing a rebuild is pretty straight forward and most folks would have no problem doing so if need be. in any case i would take apart a used unit to inspect the contacts and clean them if need be.
the oem's will tell you to never file the contacts, while it probably is not a good idea, they can be dressed up just fine with a fine point file
just keep in mind you want to keep the proper profile, generally a bit of a dome or convex shape to the contact point. they are designed to be self cleaning with a wiping action when activated, so keeping the profile is key to refiling and having a good result.
i use them in my switch gear, and have several sizes
they come in at least five common sizes, type 1, 2, 3, 4 and type 5
type 5 is a rather large piece of equipment able to handle something in
excess of 50 amps per pole iirc, (it might be higher than that i don't recall)
a type 2 reversing contactor would probably handle your needs quite well.
bob g