I think you would be better advised to ask in a UK visa advice group about her gaining citizenship there, but I am pretty sure your method (spouse visa several times) is the only route, I don't think there's an easy solution.
Thai ID card question depends what name is on your marriage certificate? I believe all of her documents should match (Passport/Marriage Certificate/Thai ID card), if they don't it could be tricky applying for visas. Good luck, I hope you get it sorted.
my understanding is that if you use an agent the first year, you have to use an agent for subsequent years (they somehow mark you down as gullible I guess), is that not correct?
If he gets the non o before he arrives he's only applying for the extension inside Thailand, which will be the same the second year.
I was replying to the person that posted. It seems that opening a bank account on a tourist visa is virtually impossible now without using an agent. It took a lot of persistence a couple of years ago when I did it, but it's even more challenging now.
I do agree with you that people seem to use agents for things that are very simple though.
With both the situations you described, I presume you pay the agent on the day (certainly opening a bank account), so asking online for recommendations seems the obvious method as there's little risk. I did it on my own, but that's what I would do now. If they asked for money in advance I would walk away.