Basically the corrosion restricts the flow of water overtime giving you very little water pressure.

This is likely caused by corrosion. 

But chlorine is a corrosive chemical, and some of the components on the roof may not like it so much (Rubber gaskets, galvanized nails)Detergents also act as surfactants.

Bleach is also corrosive and contains dioxins.In your country the OSHA and EPA no longer endorse the use of bleach for mould remediation.Hydrogen peroxide is not the be-all-end-all mould treatment, it is one of the more effective products available to those of us who are not professional remediation services.

The plumber was able to pull up a clog the first time (the tenants were not there long enough to have made this, we don't believe) and the second time we had it scoped with a camera to reveal some corrosion and damage to the sewage line...We were prepared for cap ex and have a cash reserve but still sucks ha!

Second, while Rob K is correct that there could be cathodic corrosion occurring that usually requires a moist environment which inside a house would indicate other problems.

Of course the pigtails have to be done right too otherwise you have the same corrosion issue at the splice.

The inspector recommended a water quality test as well as replacement of the main water line.Severe corrosion was noted in the house main electric panel and at the lug connections in the main panel.

I'm curious, though, if that would be corrosive to the various pipe materials (cast iron, clay, orangeburg)This could be more drastic but not as expensive, but can you simply cut the trees down near your line?  

They tend to be corrosive to wiring/pipes/etc and reported to cause respiratory problems.