The way you explain how the four of you became such good friends make me say that you should not give up yet. Give her some space to figure out her feelings. Go back to the soft swap for awhile and see if the friendship still holds out - then even if nothing else happens, you at least have some new friends! But maybe at sometime during some soft swap she'll figure out the difference between love and sex - and will agree to try the full swap again. But give her some time. She needs to be comfortable with it.
I agree that she should have said 'hold on - let's slow down', but she didn't. Just like Shelly said with her experience, she decided to go with the flow and not wreck it for everyone else. I think that just shows that she is trying hard to be comfortable with the whole situation. I don't doubt that I would do the same thing - fake it to keep it going for the others. That's not necessarily the RIGHT thing to do, but I can see it happening often!
So... good luck. I think the friendship you talked about might be enough to make this work in the long run. It's just how long does the 'long run' take? You might decide to move on, which would also be understandable. But I think, from your description, she simply needs some time.
Keep us all informed!
Sarah