1) The difference is the keyboards. Triton=61 keys, Triton Pro=76 keys, Triton Pro X=88 weighted keys.
2) It is quite good. It has graphic editing and looping, all done by the touch screen. It does not have time streching, or automatic looping or cross-fade looping, however. The sampling functions could be compared to the Yamaha EX5.
3) The vocoder is one of the insert effects, and can effect an incoming audio signal. (I couldn't even tell you a model of a dedicated vocoder, since they lost popularity in the 70's.
4) Yes. The Triton can record any incoming MIDI to the sequencer, such as real-time patterns. It should be noted that you'll still need the MC-303 to make the sounds, because the sounds (for the most part) are different (and better!) and the drum kits are layed out differently across the keyboard. You can also use the Triton's disk drive as a storage device for your other units with the bulk dump storage facility.
5) The arpeggiator can both send and receive MIDI clock, so you can control it either way.