Is multi attack still reduced by one damage? Can it be used to activate multi attack again? The new PG omitted quite a bit and now it's confusing.

My shop is currently dabating how this works, and we are not conning up with a good reason why it can't be an endless loop of multi attack

Th best way to find out how it works is to read the ability and associated text:

Pretty straightforward.

This one explains itself, Multiattack can't trigger another iteration of Multiattack.

A new change to the ability. The first attack won't inherently have -1 to damage, but a second attack would if both actions of a Multiattack were to take combat actions or power/move actions that included attacks. There are now times where even the first attack in a Multiattack has -3 to damage because of earlier attacks you made in the turn.