Native usage does vary a little; I'll tell you how I use these terms, and then perhaps other members will explain their usage.
Let's suppose that today is Saturday April 15th:
1. Last Monday: = April 10th.
2. This Monday: 17th.
3. Next Monday: 17th or (more probably) 24th. I would have to specify.
4. A week on Monday: 24th.
5. Two weeks on Monday: May 1st.
6. A week ago last Monday: 3rd.
7. The Monday after next: either 24th or (more probably) May 1st. Again, I would have to specify.
But as I say, other speakers may have other interpretations.