@Mittineague is right, it’d be easier, and not require any coding, to merge the data and then run one of the other of those import scripts.
The logic in the solution that you describe seems pretty good. Implementing that, if you are “not code-minded”, is not something that can be explained here. That kind of work is fairly fussy, and the devil is in the details.
How to deal with a user with multiple email addresses and how to merge them comes up here with some frequency, and there is a move afoot to do that. It doesn’t really happen that often in the process of importing, in my experience.
If you have a budget for this work, post in marketplace and someone will likely be available to help.