DTV is a glorified tourist visa, you can't live inside Thailand year round like you can with a non-immigrant visa, you have difficulty opening bank accounts, you can't get a 5 year driving licence and so on
For the extension you need to show you still qualify, so money in the bank, signed up for an activity, etc. Basically similar to the original application. A border bounce is much easier
If you have a valid lease with your landlords details on it including house registration etc you might be able to do it yourself. Generally you need a TM30 for any interaction with immigration including doing 90 day reports
something along those lines but I'd forget photography and friends. It's meant to be a business. I'd just go with remote editing of of images, the sale of prints from a library of existing images, and so on
if you are inside Thailand for 90 consecutive days from your last report (or entry) you are required to report. Whether you do or not is up to you. The fine is only 2k baht even if you are here for a year without reporting