@Vezelay Yes, I normally use Decovil heavy. I don't think this one was - it was some 'random interfacing I had back when I bought anything and everything and didn't label it
but definitely a similar weight. Tweed makes it a lot thicker/bulkier as well.
The studs are rivets. I got a rivet press for Christmas - the best £40 solid lump of metal I have been bought. It makes putting them in so much easier rather than using a hand punch and hammer. I can do a rivet in about 45 seconds now. This was a way of me testing it out properly as well.
The curved flap addendum to the pattern is available for free if you are a member of her Facebook group (although to be honest it would be pretty easy to draft yourself)