I'll skip the long intro about nurturing theory and get straight to the point for once
Using 2 triggers is always an OR condition, so where you are saying "Added to Opportunity" - That's a trigger, regardless of the stage it's at.
Where you are including the field change for the opportunity stage is a separate trigger altogether.
It seems as though what you're looking for would be something akin to:
Trigger: Added to Opportunity
Trigger: Data Value Changes is Opportunity Stage - New Value is A1
Filter: Opportunity Stage is A1
It's not pretty, but it will do.
The important takeaway from this: Triggers are ALWAYS OR, and you can't build any rules for how they relate to one another.
i would go one step further and use a recurring batch with the changes in Past X Day to reduce the load on the system.
You actually want Not Was Sent Email to prevent the lead from getting the same email. You may want to consider Engagements to avoid this issue altogether. It's a longer conversation.
Also here's the question, say we did go the Engagement route, how would that look compared to this? Setup wise? And we are looking at sending the emails to only one target person on the opportunity, so how would be make sure that only that person would be receiving these emails when they move stage to stage?