This Monday...

I would say that "this Monday" works best if you point down at the ground when you say it. Dunno why.

I doubt that helps.

Just as you've pointed out, it leads to confusion since it's typical to refer to the present day as "today."

So ambiguous, Geo, hardly anybody would say it. Like you suggest, we'd better say "Today"

Both ambiguous, equivocal, yet used by everyone at all times, one of my pet peeves

"Two Mondays from now" would be best, but nobody says it this way

If it's Sunday we might say "a week from tomorrow," but if it's Saturday all bets are off

If today is Monday, then we would never refer to it as "this Monday", but only as "today".
"This Monday" means the next (upcoming) Monday.

If, on Monday, you say "I have a meeting for next Tuesday", you are not referring to the next day, but the following Tuesday, eight days hence.

I hope this helps.

But Mac, suppose it's Wed or Thu

Keep taking the tablets, Dale.

Coincidentally Mac, this morning they were all stuck together, nearly choked me to death

...for which some of us might wish to happen