Hypersmurf
Moderatarrrrh...
Since you can substitute a trip attack for a melee attack, he makes Trip attacks against every opponent within 5' (a touch attack) for the Whirlwind attack. He then follows up (against each one he dropped) with a melee attack due to Improved trip...
Almost. He'd take each improved Trip attack immediately after the Trip that dropped the opponent, not at the conclusion of the Whirlwind.
Like Cleave attacks - the slot into the Whirlwind sequence, not afterwards.
-Hyp.