Go visit your local immigration office and ask these questions. As for your 12 month visa, the only option is an ED visa. A semi work around would be to return to the UK, get a 6-month multi entry tourist visa that you can stretch to 9 months, after which you could try to get a regular 60-day visa from Penang or Cambodia, which is extendable to 90.
That's certainly not the norm. Of course, there are exceptions, and I was in that category, as my now ex-wife was wealthy (through inheritance), but she definitively stuck out in the community as the "richer than her husband" woman. In the VAST majority of cases, Thai wives come from poor backgrounds and seldomly work at all. That said, a "good income" in Thailand is a pittance of maybe 15 or 20K. In fact, marrying a foreigner is akin to escaping poverty.
Why do you ask a Vietnam immigration related question on a Thailand group? There are a few FB Vietnam dedicated only to visa questions. That said, there's a cottage industry targeting tens of thousands of expats living in Hanoi and Ho Chi Minh who do regular border runs. I wouldn't worry.
If you're talking about obtaining a visa for the country of his or her spouse, it depends, as immigration policies vary greatly. Sometimes, it's actually more difficult to obtain a visa if the person is married to a national of the desired country to visit.