I followed proper procedure to clean up the formaldehyde, but I am wondering if I should take any further steps, or if the microwave can still be used.
I'm not sure there IS a proper procedure to clean a microwave exposed to formaldehyde. I'd clean with soap and water, of course, but I'd also consider using a household solvent like isopropanol or ethanol (er... Gray Goose?) first. Then let it air out with the door open and unplugged over night.
More than the contact exposure, heating formaldehyde in an uncontrolled environment is not a good idea because it's quite volatile. The danger of inhalation exposure during your "experiment" may exceed anything you'd get by contact or ingestion after the fact.
Obviously, this post does not constitute any medical advice. If you are truly concerned, throw the microwave out and get a new one.