writing style - Capitalizing Personal Titles as Substitutes for Names

[I am unable to write "Hello, English scholars" as the first line without it deleting it.]

There is obvious contention between style guides on the subject of capitalizing personal titles when they are used in the place of a name, such as "I heard from the Minister of Justice today". The Canadian Style Writing Guide advises this usage., and so does the British MHRA.

Assuming that this convention is being accepted, and such proper titles are capitalized in this way, I am still curious about substituting names with non-formal common titles, such as "The assistant gave it to us today". The position of assistant does not bestow a title, and so it would be lowercase preceding the name; however, in this hypothetical context, it is understood that there is only one assistant. The question is whether to capitalize "assistant" when it is being used as a name substitute, or leave it as lowercase.

For reference, all of these are correct in my setting:

• I bumped into the Professor yesterday. [Referencing a single professor, in substitute of their name.]

• You will be introduced to a professor soon. [Lowercase because of the general reference to "a" professor.]

• Hello, class. My name is Professor James. [Capitalized because it is used as a title with the name.]

• His assistant is named Julie. ["Assistant" is lowercase because of the possessive.]
(EDIT: Bad example. Here's a replacement: "I'll go ask our assistant."

• Hey, Assistant! Come over here [Capitalized because of the direct address.]

I'm hoping we might have some useful discussion on this, pulling reasons for the varying styles from guides. I am constructing a short document for my workplace to serve as a style guide on a few contentious grammar/mechanics points.

Note: Mods, please don't mark this as a duplicate. There is no other substantive post with this specific question. :)