Immigration offices have "checklists" they can give you for requirements. I recommend getting one from the office. Also, I personally value my visa and permission to stay here so compliance is very important to me. If you miss 90-day check-ins, and if you value your stay here, I'd recommend looking harder at compliance. There's too much left to the discretion of the immigration officer you deal with to be going in for a major event like an annual extension and have all sorts of gaps.
Just an observation from a friend, so take it with a grain of salt, but he is on a retirement visa so he had to have the requisite coverage and got a policy for 36,000 THB per year. He had a slip and fall and broke his leg. Had to have 2 weeks in hospital, hip replacement surgery, and another week in rehab at the local hospital. Then he found out the deductible/excess on his policy was 1,000 000 baht and his costs were below that so it was entirely out-of-pocket. Read the fine print on any policy.
I downloaded a sworn affidavit of eligibility to marry, signed it, and had it officially translated by a thai translator. I don't recall the embassy being involved.