Nice one Jeff!
I think you're right that the barrel has been turned down at the muzzle so the original banded front sight could be refitted. The barrel is a fair bit bigger OD at 22" compared to the original 25", and some modification would be required to re-use the original fore sight.
You're also correct that the plunger/button is to allow the front sight to be removed. Just push down on the button and the sight can slide forward and out of its dovetail.
It appears a single standing rearsight has been made and installed in place of the original standing and folding one.
Looks like a re-stock, which should be a good thing so long as the bedding is done well at both recoil lugs.
It also has the good "standard" type trigger, which is a real bonus IMO compared to that horrible BRNO set trigger.

Should work out fine. Nice rifle, and thanks for the photos.