Those issues aside, it make sense to just cut in a tee after the PRV and run a 1" PEX line over to the other apartment. That could just go to a new tee cut in somewhere in that apartment's distribution. That would leave you with a loop in the cold water line, which is fine and gives less pressure drop, but could be a bit confusing as far as shutoffs. So you'd want a ball valve at the supply end of the new run, and perhaps a ball valve in the current supply line to the apartment. The latter you could leave turned off if you want to avoid a loop.
That sounds like a plan right there. Thanks! However, my crimp tool won't do 1". Got to stick with 3/4.
But wait! That would mix filtered water with unfiltered. I can't use a loop, and I will have to have a second filter.