I know the physical difference between one and two piece keycaps.
Is there any particular benefit of one over the other?
I'd say the two piece are better as you can remove and clean the top bit without removing the stem and having to reinsert it correctly over the spring afterwards.
Have all SSK got one piece caps?
Some good questions for CPTbadass's SQSA thread
1. not really. Like most things the differences well make one over the other better for a certain person based on their preferences and usage scenario
2. Personally, I prefer 2 piece so I can swap my KBs back to QWERTY for my roommates to use quickly. Last time I mis-seated the "B" key and he wouldn't use it.
3. I'm no SSK expert, but I think some have two-piece, like the case for most IBM keyboards.