If you've got 400k seasoned for 2 months before you need to apply for marriage extention there's no need to border bounce, just change the reason for your extention of stay.
The original visa expires after 90 days, you'll then be on a 1 year extention of stay, which you can re apply for every year without having to leave the Country, no need to buy another visa.
There's no need to leave but you're not renewing the visa, you'll be applying for a 1 year extention of stay after the initial 90 days of the Non Imm O
You have to pay for the course/classes, it's up to you if you attend or not. If you dont attend and Immigration get wind of it then you'll most likely have you visa/extention of stay cancelled. A reputable gym probably wont even supply the paperwork for an extention of stay unless you attend classes.