1) That's already the case. But the line of sight between the character and the target (the shortest path) can be completely different from the actual path, mostly when buildings come into play (with a blocked door). But it doesn't happen very often, and is not easy to exploit.
3) I also feel a bit uncomfortable about this, but this reaction is only active on demand for a few seconds and, of course, only for one single character. Though it would be useful to measure the influence of this reaction on performance. Epic Encounters is quite scripting-heavy, so it's a reasonable concern.
