Estimating on the basis of our
apple trees these seem maybe about 7 years old. If you find out for sure that they are much older then they are really not growing so well.
The white splotches (is this what you meant by mysterious blight?) are lichen, telling you that air quality is good; but possibly also that the trees are not in full vigor. Lichen does not harm the tree.
The tree on the first photo seems OK to me, the second very questionable, the third could go either way (bark seems to be quite cracked; maybe post a pic of the entire tree), the fourth OK again. Personally I would dig out #2 as it seems really weak. As to #3, in my developing
experience apple trees often look like zombies but as long as they are of good size keep bearing quite well.
(And now you can tell me that photos #2 and #3 are of the same tree
Does your soil have any special properties like being unusually wet / heavy / extreme pH ?