Internal corrosion in the lines can and will affect the pressure and draining capabilities of the plumbing system at some point in the future.

He could tell by popping off the doors they the insulation was falling off (sparks flied at one point) and the evaporator condenser was heavily corrosive which can leak harmful R22 refrigerant into the home plus found bio growth inside leading to potential health concerns.

The second ball valves may just need snugged a little ... that usually stops a leak from that location.The third looks like the joint below the valve is where the leak is/was.The first may just be the bleeder ... you may be able to snug it as well, but with that corrosion, I'm not sure.Those valves should have been checked shortly after installation (a few weeks) and then periodically as part of your annual, pre-heating season maintenance.

I showed him the dust & corrosion on the window Al capping & he was still not convinced.

The last electrician who came to give an estimate regarding electrical upgrading for installing washers and dryers (another post) said that it was urgent that we replace the fuses and main breaker which, he said, had evidence of "chuckling" or something like that...meaning some green colored corrosion.  

What a relief I felt reading your reply:)Quick clarification: For the Schedule K copper, likely 1" line, silver soldered where underground, for water main and irrigation, with brass valves, wrapped in black foam jacket before they backfill- Does the copper need a plastic sleeve too to prevent potential corrosion from the soil (or from grass fertilizers/pesticides/stuff in lawn/garden), or is that what the black foam jacket is for? 

Power trains don't wear out like they used to and corrosion is the real enemy.

I had a plumber somewhat recently say that due to corrosion and the age of the duplex, he estimates I'd have to re-pipe both units in 5+ years, briefly estimating the job to cost $12k PER unit. 

Yes, pee is so corrosive that it destroyed the condenser coil.

@Ola DantisLeaks are normally at the evaporators due to the condensation over the years and corrosion.