If you are inside Thailand for 90 consecutive days you are obliged to report. The grace period only applies to the report not the number of days inside Thailand
even if your immigration office allows you to use overseas income for an extension of stay based on marriage you need to transfer the money from overseas monthly, you can't miss a month or transfer double one month to average out. Best to stick with the 400k but you don't need to keep it in the bank all year. Once you have your extension you can withdraw it as long as you replace it 2 months before you apply for the next extension