The bit about moving through an enemy's square does seem a little too easy to me, but it hasn't ever come up IMC. It's nearly always easier to go around instead of through the enemy. On the rare occasion that a BBEG needs to block a narrow passage or doorway, he doesn't just stand in it, he uses magic or (oddly enough) closes the door.
As for avoiding AoO, I don't mind the idea of a static Tumble DC. IMO it's similar enough to a mage casting defensively. A mage casting magic missile in combat always rolls against DC 16, regardless of whether he's facing a Com1 or a Ftr100. By the same reasoning, I just consider the tumbler to be "moving defensively," and avoid thinking too hard about it.
I don't think easy tumbling changes tactics very much, either. If the monk wants to dash past the enemy melee line, he's going to go past. Unless the baddie can take him down with a single AoO, he will get through and threaten the back-line caster, and you aren't very well going to stop him. (Attempting a Trip as an AoO can work, but it's risky. A low roll can put the meleer on the ground himself, or at least force him to drop his weapon.)
As for avoiding AoO, I don't mind the idea of a static Tumble DC. IMO it's similar enough to a mage casting defensively. A mage casting magic missile in combat always rolls against DC 16, regardless of whether he's facing a Com1 or a Ftr100. By the same reasoning, I just consider the tumbler to be "moving defensively," and avoid thinking too hard about it.

I don't think easy tumbling changes tactics very much, either. If the monk wants to dash past the enemy melee line, he's going to go past. Unless the baddie can take him down with a single AoO, he will get through and threaten the back-line caster, and you aren't very well going to stop him. (Attempting a Trip as an AoO can work, but it's risky. A low roll can put the meleer on the ground himself, or at least force him to drop his weapon.)