Then there were the 3/8 Cu feeds off 3/4 inch main lines so corrosion jammed them up over the years.

If you are close to a beach, is there salt in the air that can speed up corrosion? 

If you have a tenant that is dealing with corrosive chemicals, it can eventually eat away at framing members so be careful on who you lease to and what their plans are.

Using the snake through the roof stack would still put us at the same clog.I don't know if it would be possible to open up the access panel and try to disconnect the drains that way and get something in the pipe to try to grind out the corrosion. 

Look for spots with corrosion of the pipes rotting from inside (main stack can cost upwards $4000 if you don't do your own demo).

Without a pic of the faucet, I have no judgment on that decision, but if showing excessive corrosion and/or age of plastic components, depending on overall condition of the unit I will likely change as well.

If there's no rust or corrosion or any other sort of water damage on the electrical components then I'd say leave it be... it's leftover from a previous symptom.

This would be most noticeable on things like electric ovens and clothes dryers that draw a lot of current.Too much current thru too small a wire causes corrosion, especially at junctions with dis-similar metals - Like the connection at the circuit breaker. 

We are required to provide them in the stairwells/hallways (on each floor) in the smaller buildings which do not have sprinkler systems.We also provide an 5lb ABC (low corrosion) type fire-extinguisher in each of our units - usually in plain site in the kitchen.   

@Chris AdamsI doubt you can get someone to remove all the drywall and reinstall for 7,000.Also CDW was corrosive do most likely your electrical at a min would need to be redone