You cannot get a DTV inside of Thailand. It's only available from Thai embassies through the e-visa system.
It's also not as simple as just applying. You need to qualify first, and then provide all the paperwork as part of your application showing that you qualify.
Make sure you get the proper paperwork from your employee stating the end of your contract and last day of work, because you will need to go to immigration yourself with your passport and cancel your work extension to properly close out the loop. If you don't do that, you could be limited on visa options in the future and fined up to 20,000 baht for failing to notify immigration that you stopped working. Even when your extension ends, you are still required to report the change of status to immigration and officially cancel your extension with them. You can do this a few days early and they'll still let you stay until the date in your paperwork if it is on or before your extension ending.
"can" see is not the same as seeing. Why take the risk when handing them your passport, and also checking your stamp, will help both them and yourself.
Because no marriage extension is handled at the immigration office. They have to be sent to the district office for approval. That signature costs a lot more than the one from your local immigration office.
This is the reason marriage extension always has an "under consideration" period where you have to return later. Everything is sent to the district office.
For retirement extension, everything is handled instantly and locally.