your wedding has nothing to do with it. It's 400k thb usually in a home bank account for 3 months prior to applying for a Non O visa. Once you have that visa and go to Thailand (within 30 days) you have around 4 weeks to then put it in a Thai bank for it to season for 8 weeks before then applying for a 12 month extension (not visa) of your original visa. You have to do that every 12 months. If you leave Thailand you need a re entry permit or you loose your extension rights. You get that at the airport just past security and get a stamp in your passport. Single re entry or multiple re entry options you have.
Yep, you go out using the UK then mid air change over to Thai passport. Both passports will be linked as a dual national. Straight through the Thai line at BKK though most are mixed now anyway I find.
You are recommended to apply for a visa at least 4-8 weeks before the travel date but do not submit your application more than 3 months before your departure date.