"My wife is marvelous."
Translation:Tha mo bhean mìorbhaileach.
It seems to involve the verb "is." It separates "wife" from "marvelous," and so "mìorbhaileach" has no lenition. But "bhean mhìorbhaileach" is a unit and both words have lenition. -- That's a very layman explanation, and purely based upon observation, so I could be wrong.
You are correct. The literal translation for the sentence construction in Gàidhlig is something like: "It is the case marvelous - my wife". I think the technical grammatical term is "predicative modifier". The agreement of the"mìorbhaileach" modifier is with "tha" in this case, rather than "bhean".
Because you're not saying 'my marvellous wife' you're saying 'my wife is marvellous'. Lenition only happens when the adjective is connected directly to the noun as a unit. If you wanted to say 'my marvellous wife is small' then it would be 'tha mo bhean mhìorbhaileach beag'