Hi,
Yes and no for the first one.
Yes it does seem the numbers now need the full number including Country Code, however there are Automator and AppleScript items out there that will do it in one go.
It does depend on which version of Messages (and the Mac OS) that you have.
The full name of the .plist is com.apple.ids.service.com.apple.madrid.plist
It seems that is some Updates and Upgrades this remains in place and seems to be readable by the app.
It does not seem to get updated daily from the OS as it used to, as the OS works out where your Mac is - but the info about the Country still exists in this file.
It is when the App loses contact with this file - normally at Updates or Upgrades that the issue presents itself.
It does seem that it one of those things that does not seem to effect everyone but appears when the OS is updated (OS 10.13.3 to 10.13.4 for instance) or Upgraded (OS 10.12.x to 10.13.x)
Not a perfect answer I know but it is best that we can determine from the posts on this subject.
9:42 pm Tuesday; March 6, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)