No issues what so ever. Your visa let's you come and go as many times as you like for 5 years. You will be stamped in for 180 days. Just show your visa and passport and check the stamp before leaving the immigration area. Do not forget to complete your TDAC upto 72 hours before arriving.
If you go to enough branches, you will eventually find one that doesn't know a DTV is a tourist visa. I opened my first bank account with a visa-exempt stamp. The girl said she needed a photocopy of my visa, so I pointed to the entry stamp on my passport. I still have this account. It helps to be outside of popular tourist areas.
It starts when you enter the country. You do your extension at the end of the non O 90 days, so it counts as a report, and your next 90-days is into your 12 month extension.
A single-entry tourist visa is for 60 days, but can be extended for a further 30 days at a cost of 1900 baht. A multi-entry visa is valid for 6 months, but every 60-day entry can be extended for 30 days for 1900 baht. If you time your re-entry correctly, you can stay for nearly 9 months.