that was my question as well.

Sew the stiff material to the unstretched fabric -- but make sure you are using a bra that fits you well without the stretch first. The reason why you eliminate the stretch in the bra (both the straps & the band) is so that you don't jiggle all over the place when you dance.

As for the closures, you can either sew the pretty fabric all the way up to the original closures & use them to close the bra. Or, what I prefer to do so that the bra is completely covered in the pretty fabric, is cut off the hooks & eyes that came on the bra -- but remember to add that length back in when you're sewing the stiff fabric & the pretty fabric on, or your bra back will be too short/tight. Then once it's completely covered, I add my own set of hooks & eyes (can usually be found in any fabric store) to the bra. What's nice about this option is that instead of using bra closures, I usually use flat pant/skirt hooks & eyes which (a) lay more nicely in my opinion, and (b) hold my well-endowed girls in more securely.

It's never a dumb question! I typically do *not* stretch the sides as the elastic can give out. So, yup, I sew it on unstretched. Also, a note, I always use the stiffer fabric (it's the white fabric that I sewed on first in the tutorial) and make it a little longer than the actual side strap. Then, I cover this with fabric and it is more than enough to hook and wear.

As for the hooks, I use heavy duty pant hooks. I cover the original hooks (or just cut them off) with fabric and hand-sew on 2-3 heavy duty hooks. They hook together really well and hold fast.

As for your technique of cutting off the stretchy sides, it would depend on the look you're going for. I've found that just covering over them (and using them for a base for my stiffer fabric) means I can cover a bra super-quickly. I've done a few cabaret costumes where I've made the sides different (with cut-outs and whatnot) but still used a bit of the original fabric as a base. If you cut it all off, I think it would be a lot more time consuming to sew more on to the bra, but if it works for you go for it!