1. Could be network latency (you get the warning a second later than you normally would, for example) or it could be the other guy's firing technique - he may have launched at you from closer than you expected for example - so the kinematic way of defeating the missile (which is exactly what you're doing) doesn't work so well.
2. They have their reasons ;) It has to do with scan patterns and whatnot, and there may also be some network latency involved as well as a missile that's guiding to you acquiring you much earlier than you think it did, but diving into your RWR coverage to intercept you - when it gets in there, and you get the warning, you may not have a lot of time to do anything about it. ECM can also affect this.
3. No idea. Show a track that reproduces this problem and maybe we can figure it out. It may be possible that the RWR is switching primary threats on you.
4. Above corner, but not too much - you need to be able to change position very quickly and you can only do this at corner speed. The added speed on top of it will help you preserve corner in your maneuvers. Although, like anything else, really, it depends ;) You won't be surviving anything if the other aircraft presses on you too, esp. since he can do much more radical evasion maneuvers when his missile goes active - while you have to maintain missile lock.
And if, within 20km, he breaks this lock even once you're probably in a lot of trouble because now he's the offensive fighter and you're purely defensive (so run, or get yoru buddies to help you - or if you have terrain, use it to regain some measure of advantage, or at least go neutral)
3.