As far as brace buttons go, I'm considering them myself. Having lost some weight, my options are either wear braces, cinch my kilt up with a belt, or expose my dignity. of course the other option is to have the kilt taken in, but I don't know how to do that, and I don't know who does. Right now, the heavy-duty tool belt suspenders (found at hardware stores) works fine, but over time, I'm afraid of the damage alligator teeth will do. I would just say keep them hidden. I'm assuming if you're wearing braces, you're not belted, which might imply that you're wearing a waistcoat/jacket. If no waistcoat or jacket is employed, I would go with a belt.
Bowties are fun, and I am a fan. In fact, I think it is (or should be) a law of manhood that one posses the ability to ties one's own bowtie. As long as the tie does not compete with the kilt, I think they're great. A simple regimental stripe or club pattern looks great with tweed daywear or an argyll worn dress/casually.
As far as sneakers go, I've seen it done but couldn't pull it off myself. If you're going for casual, I might suggest a loafer or some other slip-on (crocs?). If you're going for athletic, maybe a high-top boot with ankle support. Either way, I would suggest hose - even if they're scrunched down a bit on top. Maybe pass on the flashes/garters and save that for dressier occasions.
Don't forget - at the end of the day, it's your kilt and what you pair it with is up to you. There is an entire forum here that will attest to the fact that there is no right or wrong - only the kilted and everyone else!
Cheers and best wishes,
KPK