Well I haven't had much time online to update this post, but as my event has been and gone, I thought I should come in and share my photos!
I ended up going with a standard buttonhole in the collar. Not super happy with it, but it's not too bad.
I used 3" horsehair braid in the hem of the lining, and left the velvet fabric as is. I decided I didn't want it as full as I had originally planned so just the braid in the lining hem was fine.
The skirt was an absolute pain to hem (predictable) and I'm not sure how, but it ended up too short on one side, so that the lining showed. And not only that, but I had sewed the (white) braid to the wrong side of the lining, so I had to run down to the fabric store at the last minute and grab some ribbon to stitch down over the braid so that it wouldn't be super visible (just don't look too closely!)
The back gaped a little bit, I had planned to do a row of 3 or 4 self cover buttons up from the waist to make it sit better. I only had 1 button left in my self cover button kit - went and bought a new packet, same brand/size/style, and the horrid things wouldn't work
I'm actually rather angry about it, as this is now the 3rd thing I've bought of this brand (the only brand available at my local/chain store) that is really terrible quality. There was no time to go searching for a different brand of buttons, so the back just had to gape
I went with a gold metal belt rather than the self fabric belt as I felt it needed something to break it up and also tone it together with my shoes and jewellery.
All in all, excepting the hem mishap, I'm happy with how it turned out! As you can see the colour is decidedly burgundy, not purple, but oh well. I got some lovely comments on it and felt beautiful!