Okay, let's skip Affiliates and Speakers. Just Members and Guests. The former can attend, but as Guests like any non-member.
Only one form as it is now. After you enter your email address (but before you submit the form), a check is done in the background to see if already used by existing Member or Guest. If discovered, a popup appears suggesting that they login first. In doing so, the form will be populated. Otherwise, they will we registered as a Guest, and a a Guest account will be created for them (details emailed to them upon registration). The popup will contain a form which will allow them to login. Once logged in, the popup will close, and the form will automatically populated. One page for everything. Yes?