Donny,
I will try to answer your questions, but for at least some of them I have to just quote what the owner's manual says.
1- Differences in the vocalizer: I cannot comment much here, since I don't sing at all and actually have never used the vocalizer in T1; the user manual says that the T2 has 60 presets, ranging from Country Quartet to Pop Vocal (preset 81 is "Thru"). The vocalizer menu (window) looks the same, both in T1 and T2.
2- Music Finder: I don't think that it can save the key of the song; I have tried to call several songs, but each time in the main screen there was no indication whatsoever of the key; this appeared as soon as I played a chord in the left section of the keyboard. The Music Finder, however, now remembers the song you called last and so you don't have to scroll every time from the first item on the list (as it was the case with T1).
3 and 4 - I cannot answer right now, since I haven't tried to play or edit a midifile yet.
5- Yes, you can play a song recorded to the hard disk and you can even import songs from an external source (like your computer) and create a playlist with them.
6- Yes, there are three USB ports: two on the rear panel (to device and to host) and one on the front panel (to device), on the right side. There are also a video output (NTSC or PAL) and an RGB output.

On a side note: the keyboard is very easy to transport, due to both its weight (14,5 kg, 15,5 with the music rest) and also its shape (it's really easy to get a hold of it due to the protruding upper part of the case).