Glad to see I’m not the only one that runs into pandoras box of complications. It appears you are over a basement area. I would consider eliminating the cast iron all the way to a convenient point in the basement with whatever appropriate PVC transitions are required.
I recently discovered Pro PEX plumbing on my project. The would be PEX-A not B and Uponor expansion fittings. It’s used extensively in Europe and apparently being rapidly adapted by commercial plumbers in the states. This would be used for all your new water supply lines . The fittings require a specialized expansion tool. The fittings will not fail , corrode and or crack in time like the classic
metal crimp type can under the right conditions.
Have you done this valve yet ?
Advanced DIY guy here. It should go without saying , but make sure once you shut off the water main that you try and back drain all the water out of the valve and or remove that faucet line to be able to vacuum or high pressure air residual water out of the valve or your reheat removal may be problematic.
I’m going to be doing nearly the same thing for a floor based toilet supply soon.
