[NorCal72]

So if I just install mini tubs at this stage I leave my suspension options open?
No frame rail mods?
I did not touch the frame rails on my 72'
Can still run full width fuel tank?
If you run a 335 rear tire and do the spring relocation kit, no.
Can use stock leaf spring suspension initially?
If you do the mini tub is pointless because you wont be able to take advantage of the extra room the mini tub will provide. The tire/wheel will be hitting the leaf(unless you do the DSE spring relocation kit). Unless you want to do the tub now and plan on suspension and a different wheel/tire combo later.

Minimal gain in tire width (until aftermarket suspension done)?
275 is what I recall being the most popular width tire on a stock GenII, I think some are getting a 295 under with some mods. With a mini tub you can do a 335 easy and I had enough room for a 345.
Will still need to mod back seat & rear interior panels?
Yes
Rear seat belt mounting point relocation?
Yes
Are there any issues/benefits I am not considering by going with just tubs at the start?
I would suggest considering your long term goal and waiting until you can do it all at once. The mini tub is all of a 3 day weekend to get done. If you are going to have everything torn down to do that you might as well do the entire thing at once (tub and suspension). IMHO

Good luck, I just got my rear suspension and tubs done, I couldn't be happier with the end result.