If a tree contracs root rot can you or are you able to save the tree by bridge grafting other root stocks?
Or if a tree I'd not doing well with root stock can you bridge graft it to revive the tree back to life
or do multiple root stocks so you can have the benefits from all the root stocks
sry for do many questions but I have had this in my head for a few days and Internet not helping me

It has been done a number of times for valuable trees as shown in the following slide presentation.

http://ecoport.org/ep?SearchType=slideshowViewSlide&slideshowId=79&slideId=1458

Start at this slide and look at the next four slides.

Inarching is actually not very difficult to preform. From the small trees that are being used as the inarch, all branches are removed and the top is cut off with a sloping cut about two inches in length. Then in the side of the larger tree that is being saved, at the proper distance above the ground an inverted T incision is made, and the corners formed by the intersecting cuts are turned back as in shield budding. The prepared end of the small inarch trees are inserted into the cut, and firmly tied in place. If it is difficult to tie, it may be nailed with a slim finishing nail, driven in with a punch to prevent bruising. In most cases the punch method is preferable. All the incisions are then carefully covered with wax. The bandage should not be disturbed until union has taken place and the parts have firmly knitted. - Millet (1,103-)

Millet are you able to answer any of my questions?

There was a thread on the benefits of multiple rootstocks a while back--I think Joe Real had some info and pictures. It can help to save the tree, but I have had trees with root rot that were saved just buy removing the conditions that were causing it.