Most meaning very sounds sophisticated to me. I also associate it with British usage (but I may be wrong here). I think someone else may be able to answer your question in grammatical terms; for me, that is the only one that sounds right here.
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"Most kind of you !" , "I'm most beholden to you, indeed ! " You may hear that in a Victorian play .
These are expressions where "most" is used to mean "very " , "much" or "extremely" .
Being rich isn't ..... important to me. Depending on the context, "that" is used here as an intensifier to mean " not as... " or " not very" as in :
-Tom will give you back your moen