With a PH as low as 6.8 being detrimental to copper pipes I would suggest an acid neutralizer be installed to protect your remaining copper pipes. I would also go with whatever other treatment is recommended per your water test.
I would suggest CPVC would probably be best in your application as the mice do have a likeing for some types of PEX in particular the brand I have pictured below... LOL
Your climate is not all that bad and as long as you take appropriate measures to prevent freezing you should be okay. Block that vent in winter and insulate the pipes. If you do get a freeze get some heat in that crawl space right away before you get a split.