That is fascinating! I had my first pregnancy right after onset and, I believe, declined through it but was much worse afterwards. My second pregnancy, however, I had hyperemesis as you did and experienced near a total remission that lasted something like two years afterwards. After my health declined to the same state I had the bright idea to have another baby so I could feel well again. It didn't work. I so declined during my third pregnancy that by the time my daughter was born I could neither sit up nor hold her. My point is, my only remission occurred in the pregnancy with hyperemesis. If only we could replicate that (without the vomiting). I would love to try it again, but at this point, I don't dare.
I do know that I needed progesterone for a luteal phase defect before my last and I don't think I ever made near the same amount of hormones because I never really went into labor after my membranes ruptured (no oxytocin) and ultimately couldn't nurse due to having low prolactin levels (they used to be high, go figure). Anyway, now I don't seem to make any hormones at all (except DHEAS!) so we tried BHRT last year and it made no difference with either the obvious hormonal problems or ME.
This hyperemesis thing makes me think of VanElzakker's theory on vagus nerve, because I understand that one of the principal effects of drugs that stimulate the vagus nerve exacerbate vomiting.