As previously mentioned, to apply for the COE you need to upload details of your flights, ASQ booking and health insurance details. If you apply a number of months in advance, you then lose those months on your entry stamp because of your insurance validity.
... maybe depends on the consulate you are applying at but I have used my UK marriage certificate 3 times to get my non O spouse visas at the consulate in Saigon. Best contact the embassy/consulate where you're applying.
... you need to do a TM30 EVERY time you leave then re-enter Thailand. The TM30 has your TM6 (landing card) number on it and you fill out a new TM6 every time you enter so the TM30 must be updated.