What they assured you and what transpires remains to be seen. The whole same sex marriage is a huge can of worms. Especially later on when you have to deal with Thai immigration.
First, wait for the law to change, then get married and then see what the visa situation is, because at the moment nobody knows what will happen with those.
Correct, as you have a VISA, you can use it up until the valid until date, so if you do a border hop a day or so before it expires, you can get nearly 15 months out of it.
2. You can use all the balance if you like, as long as it is 400k for the 2 months preceding your next extension. Wise to keep it at 400k during the 'under consideration' period too.