Not sure about the execute and entity selection. I may have made a mistake there, needs further investigation I think.

The vanishing may be something to do with the persistent setting. Just an idea.

Getting coordinates on the scoreboard is definitely useful as you can effectively reverse the process. Haven't implemented it yet but pretty confident it will work.

@Tez1010 Me again. I though that might have been what you meant, but couldn't figure out the syntax of spaces and commas. Got there now.

Regarding tping entities yet again: whether or not you can target the entitiy indefinitely in unloaded chunks seems to depend on the status of where you tp it from. So if you tp it from a chunk that then becomes unloaded the game looses it. But if you tp it from spawn chunks which never get unloaded (unless the server restarts) you can target it to your hearts content.

The reason why I'm still harping on about this is that you can use it to make a very simple /back command, if the world border is something less than 15million.
When someone dies summon a marker entity inside the spawn chunks, and then execute /teleport (not tp) for the player, using relative coordinates and adding the diameter of the world border to either the x or z coordinate. You then have a marker a known number of blocks away from the players death location, that never despawns and can be targeted until the server closes. You can then just execute another /teleport command at the marker entity, reverse the relative coordinates and hey presto the player is back where they died. No need to mess around with finding coordinates.
I did this on SP, not sure if it works on MP - I don't quite understand if you could create a different marker entity for each player that died or not.

@PlesleronTepryos I don't think that you can. From what I can tell the best you can do test if a specific player is in a certain radius.