In FL, decks are pitched at 1/4" per foot. This provides proper runoff/drainage away from pool. Hollow sounding coping means it was not installed properly or popped up. Needs to be removed and reset with thinset mixed properly and at the right temp. If its too dry or too cold, the bond will fail as yours did. I have installed brick, stone and cast concrete coping on literally hundreds off pools from Miami to Melbourne FL. The reason your sheet tile at the top is popping off is because your coping is lifting up, even if only slightly, the bond between the grout and the coping is a better one than the bond between the tile and the wall of the pool, again suggesting improper tile installation. Looks like the deck had a major settling issue, probably due to lack of proper compaction prior to pouring deck. Concrete should be touching/bonded to the coping. The builder wasn't challenger pools (now out of business) were they? If not, this was a terribly flawed build by perhaps someone who just started back then or there were not sufficient codes in place at the time with proper inspections done during the building process. NOW THE CURE;
1) Tear up the deck where it is not stable (i.e. 1 1/2 " gap areas).
2) Remove all hollow coping (maybe all of it if can't be matched)
3) Undercut all waterline tile, remove and replace with new. If Plaster is bad as I suspect, it should be sounded for hollow spots, drained, chip away loose areas, prep, bondkote .
4) Retile and reinstall new coping, repour deck where bad areas removed then replaster if it was necessary.
Failure to do all the above will be like putting a band-aid on a cut artery, you just don't do it. You are in a northern state where there is a frost line. All the more reason to get rid of silicone and replace those deck areas. Water trickles in , freezes and expands, then lifts the deck further away, This also freezes behind the loose tile and will continue popping more off. It's a MAJOR job and will not be a cheap or quick fix. Good luck.
