"Do you really want to make a $100,000 decision based on advice from a
newsgroup? "

Probably not, but asking for advice from others that may have been in
similar situations isn't a bad idea.

"You need to talk to a good lawyer"

I would do that too. And since what they are doing apparently is
affecting neighbors as well, I'd talk to them about possibly sharing
the costs of litigation. If you have 6 people to support legal action,
it becomes more feasible.

I'd also talk to your elected town officials. Go to town meetings and
address them there too so it's public. Make sure they know what's
going on. Sometimes they put things in motion and don't realize the
results. This sounds extremely unfair. On what basis did they
conclude that slate can't be found that would match close enough to the
existing roof?