They're wild 'feral' - descendants of cultivated trees - and their fruit will resemble that of a parent that was grafted on, not the rootstock. If the graft was a hybrid, the offspring may be like either of the originals or sterile. The rootstock has no part in the reproductive process.
No. Heirloom plants have been propagated through pollination by the same variety for generations. That doesn't mean they were not all grafted onto hardier rootstock: all that matters are the flowers, fruit and seed.
No, they would have to breed true in order to be considered a varietal.
Does what happen? Genetic deviation? Yes, in all species of plants and animals. But these mutations are uncommon, sometimes unnoticeable, sometimes detrimental, sometimes beneficial. Botanists look for the beneficial deviants and use them for pollinators, so as to improve a strain or create a new variant.
Both would be grafted onto a hardy quince or other specially designed pome fruit tree. Peaches and plums would be grafted onto a suitable stone fruit tree. Yes, you can have several different species on one trunk.