Question: legally through 501(c)(3) law can the president be a co-signer on a promissory note between the organization and another party?

Certainly. Happens all the time when the corporation doesn't have sufficient credit reliablility.

By the way, 501 (c) (3) is a provision of the federal tax code. Corporate law in general, including the law governing non-profit corporations, is a matter of state law.