Being hypothyroid can definately play a role in miscarriages. I had 3 before I was medically diagnosed. All three babies were OK, I simply wasn't producing enough progestrone to carry the pregnancy.
After I was started on synthroid ( requested Armour thyroid 2 months later and my doctor was compliant) I carried the next pregnancy. I still had to take progesterone shots from 28th day till 10 weeks. At least the correct diagnoses and prescription kept my levels from dropping so fast we couldn't save the pregnancy.
The only other pregnancy I carried was after the 1st miscarriage when I went on glandular thyroid support on my own. At the 8 week checkup I indicated the supplementation and my doctor had me get off the raw thyroid. I did as he said and I consider myself lucky, because I carried that pregnancy. I can only guess that the hormone change kept my thyroid functioning.
After 2 more miscarriages my doctor decided to check into hypothyroidism and -- Bingo --my problem..
I was 39 when I delivered my 1st child and 41 for the second. Had I gotten pregnant before I was 32 I don't think I would have had the problems. My hypothyroid went into play noticably when I was about 32.
Unknowingly I was sort of following the ER4YT diet fairly close with the 1st pregnancy I carried. I also only ate one food for a meal. I understand this also helps the digestive system. And yes I ate several times a day to keep away the nausea. But the avoid foods for type O are primarily what made me feel bad. The doctor, everyone kept saying eat crackers ...that didn't help, that made it worse (wheat of course).
Everything you've indicated are definate possibilities from where I stand. Feel free to e-mail me if you want to discuss further.