Tikka's are a b**** to remove as noted, I've done a few and haven't needed to trash a barrel yet.
A relief cut ahead of the receiver ring will not damage the receiver. As the OP noted, what his smith did (facing the front of the receiver to remove the radius) is fairly common- but it should definitely have been discussed first.
You stated the smith "squared" the existing receiver threads. If he simply cleaned them up- without cutting them oversize- there's no issue with you still being able to use a prefit. It might look a bit odd against the now larger receiver ring face, but that's cosmetics.
If he cut them .010 oversize, thread fitment with a prefit will be too loose.
A relief cut ahead of the receiver ring will not damage the receiver. As the OP noted, what his smith did (facing the front of the receiver to remove the radius) is fairly common- but it should definitely have been discussed first.
You stated the smith "squared" the existing receiver threads. If he simply cleaned them up- without cutting them oversize- there's no issue with you still being able to use a prefit. It might look a bit odd against the now larger receiver ring face, but that's cosmetics.
If he cut them .010 oversize, thread fitment with a prefit will be too loose.