In light of the fact that this formed on the day of my warranty expiring.. How risky is it to fly with this crack? Are there any fixes out there that work well without buying a new shell??

I've been flying mine since they appeared. Tried some sort of super glue to stop them growing without luck in some spots. You can keep flying it but you may consider getting a shell replacement, of course, DJI Doesn't care if you change it or not as long you give them more money.

It also looks to be in a place where you could wrap some duct tape tightly around the arm to keep the crack from spreading or widening.