The width of the ski is not about making them easier off piste but merely a reflection of the how skis changed in order to make them easier to turn. Wider tail and shovel = easier to turn. The actual surface area of the base of the ski is not actually that different to old skis, its just profiled differently.
Powder skis will be wider in the middle, under the foot too, so they do get greater surface area.
All skis work off piste, and in fact you are less reliant on the cut of the ski. Off piste is all about technique, but modern off piste skis allow less technically proficient skiers to ski off piste
"Having said that, I will be testing the release at the beginning of each day I use the old straight skis and will start easy and build up..."
Why bindings are very simple things, basically just springs on pivots. As long as the driction plates are not too warn you should have no problems, ok the springs may have lost some strength over the years..