Yes, supplemental quant courses sound like a great idea in your case. For top business schools, it's crucial to show that you will be able to "hit the ground running" with the rest of the incoming class, so if your GMAT is bogging you down, an online course in Calc or Stats would be a good option. The Harvard HBX wouldn't be my choice in your situation, despite the name-dropping value; UCLA Anderson's Extension courses are widely respected, and have been effective for my past clients in similar situations. Berkeley Haas also has online courses in Stats or Calc, but some clients have complained that there was not enough professor support, so that's something you would want to check out. I wouldn't recommend EdX courses -- too free-form -- but it doesn't necessarily have to be a "name brand" university, nor do you have to have a grade before you submit. The most important thing is not to add another A to your transcripts, but rather to show that you are doing whatever it takes to improve your candidacy to top MBA programs.

Good luck!
Margaret Strother