Heh. I'm guilty. I fun idea for sure.
If you're serious about writing the rules, you need to clarify some:
Movement: If he's a random event, he's deployed in the center of the board. In any given turn, he will move towards the closest model that he can see, charging if possible. If there are no models within sight, he rugs in random direction (roll an artillery dice each random move.)
Swords: Why not give him skills? Expert Swordsman makes sense. Also, let him parry and reroll with two swords (he's a DP of sorts, after all).
Dog: Mention "uses the Witch Hunter wardog stats".
Collapse: Have to elaborate. "Groo moves during his own turn. If, at the end of any of his movement phases, Groo ends a Movement phase in a building that he was in at the end of his previous movement phase (i.e. been in the same building two turns in a row), the building collapses. All models within the building except for Groo take 1D3 S4 hits due to falling debris. Models that a second floor or higher must also roll for fall damage as normal. The building is removed from the board, and the area it once occupied counts as difficult terrain."
Boat: Same as collapse.