Old Concrete Pool - Post-restoration issue?

All,

Last year I did a major waterline re-tiling on my 1970s concrete pool using researched proper thinset, etc. Upon opening I found the upper most waterline tile has popped off in many areas. I have a few questions: 1) behind the top most waterline tile there is a definite visible line where the coping meets the pool wall/beam - I may have tiled over this gap and wonder if either moisture or shifting caused the problem of ataching to two seperate surfaces and, if so, should I only replace the tile only on the pool wall and stop just below the gap (demarcation between beam and coping)?; If the answer to #1 is to not tile across this gap can I use an epoxy versus thinset for these 2w x 6l tile and if so what kind is recommended (I've heard of using 3M Marine sealant 5200...)?; 2) The gap between the uppermost waterline tile (asuming I stop below the gap described above) will be over and inch in some places - what type of caulk or material do I use to fill that gap?; 3) There is a 1"-1.5" gap between the coping and decking that was old and tired - I pulled it up and it had backing rod below. The backing rod was wet as was the sand below - should I skip the rod and replace with a self-leveling Velcuum contractor caulk which may help keeping moisture seeping below the coping?