Looking for some mechanical advice...

I have a new rear wheel w/ a supposedly "Dura Ace only" 10 spd. freehub body.

I have a bunch of new Ultegra 10 speed cassettes. I have recently been
using them on older hubs that are compatible w/ 9 or 10 speed
cassettes. In those previous instances, I used the thin spacer ring
that fits into a recess on the back of the cassette spider (behind the
largest cog).

With my new "DA only" freehub body, the spacer ring does not slide
across the new taller splines because it has too small of an inside
diameter. The splines on my Ultegra cassettes seem to match up fine
to the freehub body.

My questions:

1. Do I need that spacer if the freehub body is 10 spd only?

2. Is there such thing as a "DA 10 only" freehub, or might it actually
be "Shimano 10 spd. only" (meaning my Ultegra/105 10 cassettes would
work)?

3. Is there any design difference between Ultegra and DA 10 speed
cassettes, aside from the expensive and lighter ti cogs?

Thanks for the tips. I'm new to this fancy, newfangled 10 speed biker stuff.

Chris