Non-O multiple entry is still available, but perhaps not for much longer. If you go to Savannakhet no later than December 20, they'll issue one. After that, they will move to the e-visa system, where, for some inexplicable reason, they've decided not to offer the multiple entry version anymore.
Do NOT submit anything that mentions you want to work in Thailand (meaning for a Thai company or company based in Thailand) or with Thai entities.
Once you're over here, you could do some research and look into establishing a regional office. Upon doing this, you'll need to change over from the DTV to an appropriate visa.
No problem until an airline check-in agent demands on onward ticket. If anything it's better to exit Thailand by land and come back by air as you definitely won't need an onward ticket with a DTV visa.
I think IF you could qualify for a Thai passport one day and want to live in Thailand, then apply for it. Again, assuming you would be eligible. It can't hurt and it will only benefit you.
Of course, unless they make it much easier to apply for it, chances are, you probably won't ever qualify, but just saying.
Yes, for example China. Almost impossible to become a citizen.
However, Taiwan is straightforward. Many westerners have become Taiwanese and there's no need to be ethnically Chinese either.
Japanese citizenship is becoming more straightforward to acquire these days, though I believe they only recognize one nationality so you do have to give up your other nationalities if you want to hold Japanese nationality.
Yes, it's quite a hassle. I've met several farang who became Thai citizens, but it wasn't an easy process. Over time, it's becoming a little more streamlined, but there are no investment schemes or honorary Thai citizenship (except in very rare cases), unlike say Cambodia, where you can basically purchase citizenship, which again, is a bonus for traveling around SE Asia, not to mention living and working, or investing in Cambodia.