[FishStretcher]

I haven't done this, so take it with several grains of salt. But a few things occur to me.
The stringers attach to the inner skin of the transom, so it is by its nature more structural than the outer skin and it seems you might need/ want to add knees to the inner transom. After all, the boats hold together with rotten transom cores, so the real load path has to be thru the inner skin and stringer joints. Which means this will need reinforcement when you put it back together.

The problem of laying up on vertical surfaces is a trick, too. It seems like vacuum with resin infusion is the way to go, but that's pretty exotic for most of us.

<edit> I just noticed you can vacuum bag as you have pics of this in another thread

So if it were me, I would carefully remove the inner transom, add foam cores for knees if you want, and reinstall it. A plus is that with the trailer/bow up, you aren't glassing upside down, and there is no finish sanding. I think if you could use a slow resin, you might be able to do wet layup and then vacuum bag this phase of the install to suck the whole thing together without fasteners. Even shop vac level of vacuum would so for that. You would have to seal the top edge of the transom and drain hole, I think. I would think that leaving the top 3-6" of the inner transom skin intact would help with this.

But all this is just a thought on how to do it in the most general of terms.