Is "this Monday" or "next Monday" the correct way to refer to the very next Monday in the future? [duplicate]

There is ambiguity in both phrases. The first is only due to loose usage, the second is due to lack of explicit reference.

"Next Monday," in the strictest interpretation, means whatever Monday will happen closest in the future, excluding the one you currently occupy if it happens to be Monday. I think of it as similar to saying "the next red stepping stone;" if you are on a blue stone, you're talking about the first subsequent red stone you will encounter, while if you're on a red stone, you're talking about whatever stone you will encounter after the one you currently occupy. If the speaker intends to reference the Monday after the closest one in the future, the appropriate phrase is probably "the Monday after next."

"This Monday," however, is ambiguous. "This" could be interpreted as "this week's," "this past," or "this coming." Unless one is pointing at a square on a calendar, it would probably be best to explicitly use one of the two latter phrases to eliminate any confusion.

To me, the meaning of this Monday depends on the tense; in the past tense, I would take it to mean this past Monday, and in future tense, I would take it to mean this coming Monday.

Next Monday I take to mean the next Monday in the calendar (so between 1 and 7 days in the future), and like you, I condsider Monday week = a week on Monday, but I'm unsure how widespread this usage is.

This is always a contentious issue and it does come down to personal preference.

I would take 'last Monday' to mean the previous Monday.

'This Monday' is the very next Monday in the calendar. If it's Sunday, 'this Monday' is tomorrow.

'Next Monday' is the next but one. If it's Sunday, 'next Monday' is a week tomorrow. I too would take 'Monday week' to mean 'a week on Monday'.

As used in my local area (Great Lakes region of the USA), "Next whateverday" refers to the upcoming day of that name that's NOT in the current week. "This" would refer to the upcoming day in the current week (assuming user specified future rather than past).