> Dictionary says it's derived from the premise of a building deed, so it's not completely off.

Of course! The premises are "the matter previously stated" on a property deed.

I did a very shallow search and nothing came up, thanks for pointing me in the right direction.

>I wonder if there's a term, similar to "backronym", when the singular form of a word is derived from casual mutation from its plural?

I think premises/premise(s) might be unique in English in that it's a singular noun, pluralized by adding an s, where the plural form is a homonym with another singular noun that is dissimilar in meaning to the root singular noun. Lots of words hit 2/3, but I can't think of any others that are all 3.