Thoughts on if it's possible to succeed in math @ PhD level w/o natural ability in quant reasoning?
To know whether you can do an applied math PhD, the question to ask is: did you succeed in difficult math, science, and computer science courses in undergrad, and do you have professors in relevant fields willing to write you a strong recommendation? General aptitude tests are dramatically less informative. This is not to say that, if you’re correct that you have average quantitative reasoning ability, a math PhD might not be the wrong choice for you. It’s just that you have other, much more refined and relevant, information available to judge by.

EDIT: Based on your edits, I’d say an A in Calc III at Columbia is more informative than the IQ test. That said, you have few advanced courses-only one, once you finish real analysis. So there’s not much evidence yet of your prospects. You’re in a plausible position to apply to master’s programs, and if you get into a good one, you’ll a get a much clearer idea of yourself there.