Does the benefits of the curved lightsaber hilt only apply to Makashi style users or can any style gain the benefits ?

Any style. Makashi, though, is the style best suited for lightsaber duels, so is the one which will probably most benefit from a curved hilt (hint: in the tree there's already a talent which gives benefits when fighting 1-on-1)

The downside, of course, is that the bonus from curved hilt 'defensive' does not stack with Defensive Training.

So there's that.

Well that upgrade to W10 took a while !

Dosen't stack ? didn't see that one, do you have the page reference handy ?

It's in the full write-up for Defensive Training.

But the base attachment is just an advantage when using the hilt against a single opponent. Only the Defensive 1 Mod wouldn't stack with Defensive Training, correct?

However, if you made the hilt Superior (which adds +1 Damage and a +1 Advantage), the advantages would not stack vs. a Single opponent. I think I read that right (which would almost make a Curved Hilt unnecessary if the hilt is already Superior. Of course, the curved hilt costs less, so it has that advantage).