I agree that this was probably the case. A lot of what she said in her tweets and even some of her unearthed blog posts point to this - she said she tried a threesome at one point and didn't enjoy it, and the implication seems to be that it was her and Jared plus HER side piece, not Jared's. If even that wasn't enough to get her into it, it makes sense to think that her reasoning for this was less due to something tangible (e.g. "It's not comfortable, it's messy, etc") and more based in something emotional (e.g. "I don't want to fuck other people, I don't want Jared to fuck other people, etc"). So I believe her when she says that her proposing the open relationship was a desperation move and not something she ever truly wanted.
Plus, Holly's claim that Heidi encouraged Jared to confront his feelings for Holly always felt strange to me. From Holly's description, it sounds like Heidi told Jared he should pursue Holly, and then got "irrational" when he actually did it, but I think there's too much contradictory evidence to back that up. What I think really happened is that Heidi found out about Jared's romantic feelings for Holly, and probably told him something along the lines of "go handle it," with the obvious meaning of, "Go break it off." But since none of these people can communicate worth a damn, it was probably worded in such a way that he could deliberately misconstrue her meaning, and he and Holly are now using her poor word choice against her.
I disagree here. I think going public was her only recourse. Jared has a lot of money and much more power than Heidi, and on top of that, he tried to cut her off at the pass with his half-assed divorce announcement. To stay quiet would mean letting him get away with several months of being a manipulative, cheating dickhead. Her only real mistake was not going silent after the very first wave of fan responses came pouring in.
I think Heidi did an excellent job sharing her story in the beginning, and my guess is that it's because her first round of tweets was written beforehand, or at least mentally outlined prior to the beginning of the shitstorm. She did say in a post on her private Facebook that she was planning to go public with her story about Jared's actions, so it's not a far leap to assume she was drafting something for a while. The only one I believe was written on the spot was her very first tweet, since it's so different tonally to the rest of them - it's way more cutting and brutal in contrast to the later tweets, which seem to have been written with a more pragmatic approach. But that first tweet is absolutely brutal and pulling no punches.
But since she's now improvising her responses, they've begun to lose their cohesion, and because of that, she's losing control of the story. I'm inclined to believe it's less because she's not being truthful, and more because she's just not very good at maintaining her cool in arguments, given how we've seen her beginning to trip over her words as the weeks have gone on. In that sense, yes, I agree that she shot herself in the foot, but not because she went public - her primary slip-up was not shutting the fuck up after the shit with Jared creeping on his fans began popping up. At that point, she pretty much had it in the bag, and all she had to do was lean back and let Jared's career kill itself. Continuing to talk shit on her Twitter made the situation go from horrible to just petty.