Gas Regulators and Reducers

This is what I have and what I'm dealing with. I have a 1 1/2" copper stack coming down the wall with a "T" fitting. From the T it comes out horizontal for 2' with 1 1/2" copper pipe then goes into a threaded 1 1/2" pvc pipe that goes up to the kitchen sink after some bends and 10' of pipe. Well for some reason the 1 1/2" of copper 2' peace decided to split for no reason. So what are the best fixes for this? I was thinking to take out the copper 2' peace and threaded pvc fitting and add pvc slaved coupler with a new pvc pipe and slide the pvc pipe right into the copper T fitting. And use a 1 1/2" by 2" flexible rubber coupler fitting to keep the pvc and copper T together. Is that the right way and best way to do this? All that comes down this line is the kitchen sink and dishwasher. thanks

I just started redoing the kitchen floor, and I used the thin set mortar and backer boards first. I cleaned the thin set off the tools in the basement wash basin that is right next to the washing machine. Both drain into the same floor drain with their own PVC piping. Since that night the washer has been overflowing through its PVC pipe and the basin fills up when we wash clothes. Could there be mortar that hardened where the two pipes empty into the floor? I have zero plumbing skill or experience, so I'd rather not remove anything. Can I use muriatic acid in the drain to try and dissolve the mortar? Is there a potential that it backs up into the washer? Does muriatic acid damage PVC? Any other ideas that don't involve me taking apart the pipes? I'm worried that would have me right back on here asking how to assemble the plumbing I've destroyed.