A few approaches to consider:

  1) be fair and decide upon some value that should cover most cases, make the outliers suck it up, like some games kick those with higher pings
  2) don't be fair and base the threshold of visibility on some predictions about the movement of the entities in the following ticks, based on their probable movement speeds, as well as the ping times of the each player; the player with the higher ping value might receive the position of the other about 10 frames earlier before they round a corner - imperfect, but should still avoid ESP across the map
  3) don't be fair, base this tolerance on hidden metrics about how trustworthy each of the players is considered, based on whatever data about them you can get, a bit like hidden ELO - you can probably game or abuse this system with enough effort, but it shouldn't make a difference in the lives of most legit players, since it shouldn't matter whether a model that you're about to see was rendered 5 or 10 frames before you actually did
  4) enforce regional matchmaking by default and only show servers with acceptable ping times for your system (if any at all)

As for RTS games, that should be even simpler - most have some sort of a fog of war mechanic. Given that, you could probably come up with some data structure to represent everything that's visible to your side (like an octree) and send all of the models within it, without worrying about checking individual positions.

As for warping: the exact same way as in any online game, probably by some interpolation. If you receive a position from the server, the entity should be visible at a certain position, if you do not, then it shouldn't be visible (or maybe send the position in which it should disappear, with an additional flag). If you don't get the data for a while, handle it however you would stale data - like ARMA 3 does with entities just standing around or other games with them running in place, which is pretty funny.