Electrical – How to fix damaged wires in receptacle box in new installation

Yes. I fit steel junction boxes in places the old wires can reach. Then run EMT conduit between the panel and those boxes. No more than 3 circuits (MWBC counts as 1) in each conduit and use 3/4" conduit, for easy pulling and future expansion. NEC limits each conduit run to 9 conductors (4 circuits: grounds and MWBC neutrals do not count) unless you upsize conductors. I'd use the biggest boxes I can get, no smaller than deep 4" 2-gang boxes, because they will be full of splices. I use THHN/THWN through the conduit to the panel. Terminate grounds at the ground lug in the junction box, or run a wire. EMT is a ground path.

But try this. Leave the panel where it is, and install a sub-panel in public or tenant space. In fact, forward-think: if in the future you wanted to set up the tenant with a separate meter, what could you do now (cheaply) that would make that cheaper and easier later? Setting them up with a sub-panel easily converted into a new main panel, would be awesome all around. This isn't necessarily a large investment, mostly just forethought to not paint yourself into a corner.

Honestly I'm surprised your GC is trying to feather-bed the project with unneeded work when the sub-panel is a really worthy effort.

If you have enough wire to have 6" from the back of the junction box where you tie in a junction box would be the easiest way. Most of the time the wires are tight. then there are 2 options. #1 to pull the wire down from the outlet to your box using the wire to pull a string or the new NM cable down to the box some times the easiest is to pull a string when pulling the NM down then use the string to pull the new NM up. #2 Option would be to pull into the existing box from downstairs this can be a challenge getting the NM into the outlet box (if there is enough room in the box to make the junction). I usually use #1 It cost a few dollars more but saves in time trying to fish wires in a closed wall.