Process and model questions

My first questions are, when the involuntary contractions start to happen, am I supposed to try and maintain a squeeze for the muscle imbalance to continue or should I let go and see if it takes over? Like how much of this am I supposed to steer it on its course, or does it guide itself?

When involuntary contractions have begun you are experiencing an orgasm, it may not be particularly pleasurable at this point but it is an orgasm nonetheless. Once you are in the orgasm the mind and body need to work together to maintain the event. The bottom line is you need to experiment with both techniques and see to which method you respond best. We're all a little different, what technique works for one man may be just the opposite of what works for another.
My general advice is to allow your body to take the lead and have your mind reinforce the action via mental intent. By mental intent I mean you are not engaging your ego consciousness to exert your force of will, rather you are allowing your subconscious desire for pleasure to rise to the top of your awareness and focus on that. It is a difficult concept to explain but it is part of the idea behind my post "Just Let Go !".

I've heard of this auto-fucking thing. How does that work? Is that what takes over and causes the super O? What are you doing physically and mentally when everything is escalating?

Auto-fucking is simply the same involuntary contractions you have been experiencing become pronouncedly rhythmic and perhaps stronger in nature, they are just another varietal aspect of the orgasmic experience but are not necessarily a definitive aspect. IMHO, the involutaries, in and of themselves, do not cause a Super-O but may lead one into their Super-O. When everything is escalating, I first try to make sure my mind noise is reduced as much as possible, then allow my intent for the experience to predominate my thinking while simultaneously having my ego slip into the objective observer mind state. When I'm in the orgasm the body is doing its thing and I'm just along for the ride .

My last question is about the Progasm and the Eupho. Should I invest in a progasm in hopes of more direct contact? Or would the Eupho be better for hitting the right spot in this scenario?

What you should or shouldn't do is totally up to you. I can only tell you that each Aneros model has its own performance characteristics and which model performs well for one man may not perform well for another. It is impossible for us to predict if either of those two models will be more effective for you or not. While being hopeful, IMHO, is generally a good personal quality, in this context you may be tainting it with unacknowledged expectations and, as is frequently stated here, expectations can often just lead to frustration. If you can afford them I'd say buy both and experiment, that's the only true test procedure which will work for you.

I want something that will help push me over the edge.

Every time I hear that expression makes me cringe a little as it reflects a possible misunderstanding of the path leading to a Super-O. There really isn't an "edge" to cross over like the PONR in ejaculatory orgasms. Please see Super-O Myths & Illusions regarding 'the Edge' and a couple of other myths.
Rather than an edge, there is a seamless transition from orgasm into Super-O and no amount of pushing is going to accomplish that.
Good Vibes to You !