[jhgb]

> but they definitely modify the noun that comes after them

One of the two nouns is an appositive, but it's never been too clear to me which is which in English grammar. In my native tongue it definitely doesn't seem to be the case that Mr. modifies Smith, as our equivalent to Mr. is a common noun that is used to refer to any male human.

[munk-a]

In English the word serves double duty (and does other stuff) - "Hey Mr., can you tell me which way it is to the subway station" is a perfectly natural sentence. They can even be anonymized nouns (similar to la blanca - referring to a white house depending on context) - "Oh, I was going to wait to open the bottle until the Mrs. gets back". Lastly you've got the example I had in my prior comment "Oh sorry, I actually wanted to speak with Mrs. Smith - can you put her on the line?" where it is functionally, I think, an adjective though the dictionary considers it a noun in that usage.