Mark
Wow where to start.
First off you are right that some of the one-offs were Hobie Cats. Some were not. The 18HT got quite a bit of help from the HCA. When the HCA pulled the plug it was clear that the F18 or the 18HT would survive, but not both.
I think that you don't understand what I mean by "the problem". The problem was that Hobie Cat sailors had (in many areas) lost control of the Hobie Class Association (ie: we had a Nacra 20 sailors as the HCA Chairman). Obviously if you are the organization that runs events for everyone then everyone wants to have a say in the organization. That makes perfect sense but that was not what the Hobie Class was meant to be. The result was that a hard line had to be drawn in order for Hobie sailors (one-off Hobie's included, as well as the Wildcat I might add) to get their class back.
The result was predictable and was predicted. Most people (myself included) did not expect the "magical plateau" that you mention.
So, we have our class back and the question is how do we go forward. In my view the hard line is not so necessary anymore. We took the hit we needed to take and now we can moderate.
How do we do that? Well we have a bunch of Tiger sailors who want to race F18. Can we do that by the bylaws? I think so because we are supporting the Hobie Cat sailors (see bylaws).
What's the potential down side? In my opinion the potential downside is that the HCA would turn into the F18 class just like it turned into the North American Catamaran Association.
Do I think this will happen? No I don't. The reason is that the F18 class has reached a point where it is an established class and will stand on it's own. You see when you have two established classes a partnership is possible without one organization being absorbed into the other, that's key.

Now to answer your point about what yacht clubs the HCA should partner with, that is already starting. The first step in moderating the One-Design policy was to start counting open yacht club events as some points regattas. This has been in effect for only a year but preliminary reports are all positive.