That was my plan as well, to lay the hull on it's side for repair. I've now got such a large soft spot though, I think I need to support the inner layer to keep it from separating further from the outer layer under the weight of the injected epoxy, so I'm thinking the best approach would be to put temporary supports internal to the hull to help keep its shape while injecting the epoxy. I'm also entertaining the idea of using some kind of inflatable bladder inside the hull to keep its shape while I inject the epoxy. I agree that it is unwise to continue to press on the hull to find the extent of the soft spot. I think the best approach is to fill the existing soft area, then test the areas forward & aft to see if they are soft once this area is repaired. Looking on the inside of the hull, I haven't seen any evidence of porosity, so I'm not too concerned about that.
The soft spot as it stands covers the area outlined in permanent marker here:
https://photos.app.goo.gl/dxyShUrxhtDaTWxM6I am looking into replacement hulls in the meantime, but I'm really reticent to give up on these hulls entirely after the considerable time I've put into making them look like new (or as close as I could get them).
Stringers seem much more invasive of a repair than I'm interested in pursuing, especially now that the integrity of the entire hull is in question.