The geometry really isn't THAT non-Euclidean anyway. If you make your move entirely diagonally and you moved 6 squares you will have moved 30*sqrt(2) feet, which is about 34 feet, not even a whole square. Given that squares and fixed movement rates and such are already somewhat of an abstraction the amount by which you have to bend your perception of reality is actually kind of negligible. Same thing for the much maligned 'fire cube', a 3x3 or even 5x5 AoE would if it were a circle with the same radius still PRETTY MUCH cover the same squares. With 5x5 you MIGHT argue that the corner squares wouldn't be entirely within a circle, but again the difference is negligible enough that it really isn't straining realism to any noticeable degree.
As for the whole OA thing, listen to DS. A readied action is taken as a reaction and those happen in OTHER creatures turns, the readying creature has already taken ITS turn and there is no such thing as an action outside of a turn. AFTER that the creature changes its initiative order, but that isn't relevant to when the action happens.
So, it is quite feasible to have a pack of monsters charge past someone all at once using readied actions. However there are a few problems they will encounter doing this. Readying an action is itself a Standard Action. This means if you ready a Move Action you are basically getting a Minor Action and a Move Action during your turn, and the ability to make the readied Move Action as an Immediate Reaction in some other turn. This means you're not going to be using any powers requiring a Standard Action that turn. You CAN ready a Charge instead, but that limits you to making an MBA etc. which may not be the optimum tactics.
In any case a defender with a doorway in front of him is probably best off just standing in it. While it is possible for the first enemy to bullrush you out of the way that may not work or it may not clear the obstructing character depending on the situation. There are also other things the fighter could do, like readying his own standard action to hit one of the creatures, which being an IR and not an OA doesn't use up his chance to OA in the same turn/round. A Warlord could also ready Commander's Strike for instance to provide even another shot, etc.
Beyond that even if the monsters DON'T use the readying trick they can probably get past the fighter en masse. The first one can Bullrush him out of position for example, or just attack him with some power that can drop an effect or condition on him that makes him unable to effectively respond. In actual play this kind of thing almost never comes up as a real issue. You can contrive circumstances where it works in a seemingly absurd way, but the actual scenarios where you'll have a whole wave of minions running past the fighter is rare in the extreme in any actual game..
