I may have missed part of the conversation, but it seems like you guys are making this too complicated. All you need to use BTC to make stock trades (or any securities for that matter_) is a bank that accepts BTC, offers numbered accounts, and will let you trade securities on any exchange. Once the BTC are in the account, you're off to the races. You don't need to worry about counter-parties -- the BTC would be converted to the local currency, and then you go long or short at will.

Mack