To my understanding ...
Some additional factors which should be considered:
*) Knowing when a spell is being cast is not automatic. A check is required. Note that obtaining an AOO because of a cast spell doesn't mean that you know that a spell is being cast, just that the opponent has done something to lower their defenses. I would say that you do automatically know when your opponents defenses are down, unless there is an attempt to misdirect (via bluff).
*) Knowing what spell is being cast, even if you know that a spell is being cast, is not automatic. That requires a spellcraft check.
*) Knowing the details of the spell, for example, the target location for a fireball, is not automatic, even if you know that a fireball is being cast. (Maybe, the initial spellcraft check would tell you this.)
*) You *could perhaps* ready for "when a glowing red bead is shot by the opponent, and gets to within 30'" to "move perpendicular to the line of motion of the bead, to 30' from that line of motion", or something to that effect. That would require a ruling that the spell effect is not instantaneous.
There is a similar case of a caster shooting a fireball through a window, and a player readying to block the window with a shield as large as the window opening. Could you ready to block a window on the condition that "an arrow, ray, or projectile effect of any sort is shot at the window"?
I'm not seeing a problem with readying to attack an opponent who enters range of a reach weapon. That actually would subject the opponent with potentially two attacks, if they are charging: One for the readied attack, and one for moving out of a threatened square, assuming they must continue to move to put you within their reach.
Thx!
TomB