i think you've got yourself a pretty good installer.
The plastic pipe issue is a personal thing, as I said I'm old school, that doesn't make me right, I just prefer copper.
Your boiler contains a frost stat, if your garage drops below 4 degrees or so it'll fire it up to stop everything from freezing, now your garage is not heated so the system will carry on heating because the frost stat says so, your pipe stat says these pipes are hot and overides the frost stat thereby saving fuel.
The external stat gives an extra level of modulation and will give a tweak based upon the outside environment.
Keep his number, sounds like you've found yourself a savvy plumber.