Patch management of the Virus TI is great. If you're using a DAW (like Cubase, for example), the patches you make in Virus Control will be saved with the song so you have unlimited patches in that sense.
a) The TI 2 has 20% more DSP Power I think... the hardware is redesigned and it's lighter, otherwise it's the same machine, I believe, and the OS updates affect all TI's. If money is an issue, I'd go for a TI 1 if you can find one cheap.
b) I also own a Korg Radias and DSI Tetra... "weak" isn't the word I'd use to describe the Virus

. As for using FX to fatten it up, the EQ section on the Virus itself is enough to fatten it up. I've had great success at making old-school analogue sounds (using less and less parameters each time). I've never had to use external FX's for the Virus... although compressors and limiters will help all audio sources, I suppose.
c) The Virus Control has 16 parts, each with it's own midi channel. You have 3 hardware stereo outs on the virus itself, or 3 audio buses via USB (it might be 4 now, though). Of course, you could just solo and record each part separately once you're done composing, if you like mixing in your DAW.
d) I don't use the hardware knobs when I'm using a DAW... I use automation instead, so this isn't really an issue.
I suppose you have to ask yourself what you want. The Virus is a lot of synth, and the (major) OS updates is the best I've experienced yet. The Virus Control really makes the synth more accessible, not because it's hard to program via the hardware, but because there's no hassle of saving your patches to the hardware. It's fiddle and forget. The Radias has USB-midi and an editor for your computer, but it just isn't the same as having something like the Virus Control. You can save your patches to your own .mid banks on your computer as well.
If I had to sell all but one of my synths, I'd keep the Virus because of its width.
If you do buy a Virus, I'd suggest you keep the analogue synth for added flavour.