I would ignore the PRV on the input and calc as normal, then if your required psi is less than the set point of the PRV your good. If your required psi is greater than the set point, you can look for the psi required at the PRV and make sure that is less than the set point. If not you will have...
NFPA 25 2008
5.5.1.1 Where the original installation standard is different from the cited standard, the use of the appropriate installing standard shall be permitted.
Unless the backflow preventor is installed at the entrance, the piping run overhead will have to be approved potable water pipe, unless the water utility allows.
Coming in a little late, as far as 10" pipe goes, I have only used it where required for large fire pumps. Split that flow, it will likely be cheaper to run 2 6" mains and the fitters will thank you.