You're not leaving on a passport, just arriving. Why even show her UK passport when leaving? Use only the thai passport as long as it's valid. It's also easier when arriving in Thailand as a Thai citizen.
Well,this guy is talking about getting married,so why would he go for an Elite visa that today cost from 650k (5 years) which is a fee? It's the worst option unless you are way younger than 50 and have money to throw away.