Legal Question in Business Law in California

Yes

If the money in question is truly an unsecured loan (as opposed to a capital contribution to pay for shares), then the shareholder has a right to be treated as an unsecured creditor with respect to that money.

Yes, a shareholder can be an unsecured creditor of a corporation, and, in fact, this is rather common. When a corporation decides to terminate its existence and wind up its affairs, it must pay bona fide loans to shareholders before making distributions to shareholders. It is also likely that if the shareholder were an "insider" (i.e., a majority shareholder, a director, a major officer, etc.) the shareholder's right to repayment of his loan would be subordinate to the rights of non-insider creditors to be paid. Therefore, my answer is a highly qualified yes, and thus I mildly disagree with the preceding answers.