I honestly don’t see why you think a double major will hurt you. The double major won’t hurt you unless it means that you are so overextended in your classes that you don’t do well (as in, get a high GPA). But as long as your grades are good, double majors can have a neutral to positive effect in applying to graduate school. In your case - since your interests lie at the intersection of math and CS, and you’re interested in theoretical CS - I think it can actually be a positive, especially if you need to use advanced math in your scholarship.
So
Yes, I do think that you can quite easily explain this, and back it up by doing some work in undergrad that shows your interest in the two - like an independent study.
And of course, yes, you should plan ahead and follow your interests. We can’t predict the future, so honestly you have to do what you think is a good idea now and what you think will be the best combination. But that’s what @mathandcs is saying - take a few classes in each subject to begin with and see what shakes out.