Which creditors have the first right of refusal in a business bankruptcy case?

No, the loan could not be pulled out easily prior to bankruptcy. Your use of "underpayment" is quite confusing.
Bottom line, if the entity has so little money that the owner is loaning it money, the amount at risk is part of the risk of doing business. Your question also overlooks the role of insurance.

The question is a little confusing because it appears to conflate several issues. First, a loan to the company must be properly documented. Such transactions are almost always unsecured, leaving it almost impossible to recover in case of bankruptcy. Second, if the owner is intentionally underpaying himself or herself, they may run into trouble with the IRS or FTB. This scenario is very fact-dependent, and more information would be needed to assess possible liability to the taxing authorities. Third, paying the owner back right before bankruptcy may trigger an adverse action by the other creditors (i.e., the transfer prior to bankruptcy could be considered illegal and fraudulent). A business owner should check with a CPA about tax liability and documenting the loans, and consult an attorney in order to address any potentially improper transfers before this case becomes actionable.

No. In fact, in a typical Chapter 11 case, one of the first things creditors and the US Trustee look at is pre-petition withdrawals/payments to the owners of the business. The business can be forced to sue the owner to recover those "preferential payments."
This is a complex area of the law; you should be talking to a local bankruptcy lawyer.