Since you got no reply first time around I'll try and answer a couple of these at least:
>1. I know D100’s AF sensor isn’t as good as D1’s. But is it >good enough for tracking moving dancer? I use AF tracking >around 20% of the time for my shoot.
Very hard for me to say - I haven't tried any equivalent with the D100 yet. However I have noticed that it does occasionally 'hunt' in a situation where my D1 wouldn't have had a problem.
>2. I shoot most action shots in dim light eg ISO1600, >1/125-250 sec and f2.8-4. Does D100’s AF work well in this >condition? D1h handles this pretty well.
The sensitivity of the autofocus is rated the same for both cameras: -1 to +19 EV at ISO 100, so in theory the D100 should handle low light as well as the D1h. Also note that, like the N/F80, the D100 has an AF assist lamp which will come on in dim conditions (although you can disable it through a custom function): it's only good up to about 3m, though, and it can be obtrusive.
>3. I shoot mostly single frame but occasionally I do 3 >frames per sec for around 4-5 frames. Can D100 actually take >this up? Anyone tried this?
The D100 can shoot up to 6 Large/Fine JPEGs or 3 NEFs at 3fps.
>4. Is D100's shutter lag as terrible as F80/N80?
I haven't noticed any difference between it and my D1.
>PS If D100 could indeed do the above I suppose I’d trade my >D1h once D100’s price comes down a bit. I may indeed get >AFS17-35 & AFS80-200 to speed up the AF.
The combination of your D1h and the AF-S lenses would give you the best possible focusing speed and most accurate tracking of a moving subject.