Some Quick Questions

[Grandpa]

Newbie DM questions:

1. Bull Rush, Charge, Grab, and Opportunity Attack are listed as powers in the Rules Compendium. Is there a tactical consideration (e.g., some other power that allows you to perform an At-Will Power on condition x) to this change? I'm not familiar enough with various player options to know.
2. Bull Rush has no size restrictions in the Rules Compendium, unlike the PHB. Is this a mistake or errata?
3. Opportunity attacks are triggered by adjacent enemies leaving a threatened square. Do Large+ enemies do not trigger opportunity attacks if the threatened square they occupy is always occupied while it moves, because the monster did not "leave" the square?

For Example
Huge monster (M) vs. Player (P), monster moves from:

------
MMM---
MMM---
MMM---
----P-
------

to:

------
---MMM
---MMM
---MMM
----P-
------

No opportunity attack? Thanks!