How well does wool work during the summer? I'm not going to be sweating buckets, am I?
EDIT: Just realized it's almost September, but still plenty hot!
That all depends on where you are located. Generally, no, you won't sweat buckets. However, I have no experience with the heavyweight fabric.

2.) It seems I'm on the lower end of waist sizes (32"), which was the smallest available in the Black Watch tartan. Is there any advice I should keep in mind since I'm generally a bit skinny, unlike the caber tossing fellows I imagine you crowd to be
I'd advise you to enjoy the skinniness! A lot of folks would like to trade sizes with you.
You might discover that you have to buckle the straps fairly tightly to keep the kilt up, especially if you don't have the hips for the job (like me). And post pictures.