These are questions you need to be asking your employer. They will tell you if they can convert to non-B inside Thailand (most employers can't or won't) and if they can they will tell you how much time you need left and if you need to apply for extension or not.
Non-immigrant visa is a visa for someone who wants to reside in Thailand. A DTV only allows 180 day stays. It is not a non-immigrant visa obviously. And the law that created it very specifically calls it a tourist visa.
First, you apply for the 90-day non-O visa from the Thai embassy in your home country. You show the amount of money that your embassy requires in your home bank account to meet the requirements.
Then, you travel to Thailand. Use your non-O visa to open a bank ASAP and transfer the money into the account. The money must bein your account for 2 full months when you apply for the 1-year extension.
You will both need your own visa, and you will both need your own bank account with 800,000 baht.
That's not a thing. You don't get to pick and choose which immigration office you go to. If you're at Ekkamai then you're in Bangkok province. Impossible to say which immigration office you MUST use because you haven't told us what you're trying to do. The office you go to depends on what visa you're currently on and what you're trying to do.
First of all, there's nothing to "resolve" at the immigration office. Your visa or extension is done. The moment you left without a re-entry permit it was gone for good. You're completely starting over now with a new 90 day non-O visa.
Second, your landlord is just being lazy.
Tell them you went and stayed at a hotel who filed a TM30 for you and returned and now you need a new one. If they want to be a stickler/jerk about it, you can be one too. Demand they do a new one every time you leave and return from anywhere in Thailand.