You've really done your research and clearly have a very good understanding of the issues. I'm impressed.
I haven't yet heard a closed model that I consider very good. (KNS8400 is close, but it's too bright and its cable definitely needs to be replaced.) There are several very good open models. I can most easily recommend HD600 and K70x.
I haven't heard the K550 yet. It's gotten good reviews even from reviewers who generally dislike AKG. I have read that if your head is small, you can hardly get a good fit, but you should otherwise be fine. Note, though, that it doesn't really fold up apart from folding flat.
The Momentum sounds good; it reminds me of HD650 in its well-roundedness. But you're right about the comfort. The good side is that it weighs quite light on the head, but it has very small earcups. Because of that, I can't really recommend it.
One new headphone, which I haven't heard yet, that's getting noticed is NAD Viso HP50. (http://www.innerfidelity.com/content/i-love-these-headphonesthe-nad-viso-hp50) But again comfort is a concern because the earcups are the same width as M50, though I'm not sure about the depth. You can ask on the HP50 thread and they'll tell you.
I'm not so familiar with the many DAC/amp combos. Are you sure you need a DAC? If you need only an amp, I can recommend the Objective O2. It's a good little box that has powered everything I've tried with it, but it's larger than the other portable amps. A number of Fiio products have been well reviewed including even E5, and I don't find that hard to believe. I've met a couple of experienced Head-Fiers who are happy with E17, but I haven't tried it.