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They should branch on their own here in the next year or so. You can force them to branch now by nipping the top, but I would not recommend that as this will result in a tree that looks more like a shrub than a tree.

Unfortunately, it is unlikely the trees can be saved if they have been that severely girdled. If you are very attached to these trees, I would call a local arborist to look at them and they may be able to help.

It would most likely kill them to be cut back that severely. Plus, if it did survive and regrow, the growth it would have would not be suitable for espalier.

If the plant is still alive, then I would give it a little more time. While it is spring, tress rely on more than the temperature to tell them when to grow. For example, the amount of light a tree gets plays a large role in how and when it starts to grow. The days are getting longer and the tree should start to open its buds when the light it gets is long enough.

It is likely sap. Sap in the spring is very watery. It will be fine and this will not hurt the plant. It is common for recent pruning cuts to do this in the spring when sap runs.

Trees, like animals, have lifespans. Most apple trees live only 15-20 years, so yours is quite old and is most likely dying from old age. The fungi is most likely not causing it to die, but is simply being opportunistic and attaching to a dying tree.