My wife was in same situation, she had to go on Non B for 90 days, then moved to spouse on my Non O. This was done via Thai agent and our immigration office.
We had already tried the merry go round of KL - refused, Phnom Penh - refused. We were told to go to KL or PP by our immigration office. So we wasted about £2k doing this and still had to pay an agent.
Once she turned 50, we both got our own O-A from UK.
Depends, I got both mine at my local DLT and no tests required beyond peripheral vision test (that's the eye test, not the driving hazards one) . I did it in person.