1. A copy of the title can be demanded from DENR, but then it will be in your father's name.
2. I suppouse the lender know how much interest they had agreed on.
The lender can demand interest at the unpaid interests too, step by step when they were suppoused to be paid.
3. Both loan and taxes have to be paid to get the title transfered.
The inheritance need to be solved too to sell it. When selling it can be solved by all hiers sign a document saying they agree to the selling. (I have forgot the name at the demanded document, but DENR know,)
Checking: Not paid loan and interest long time - Are you sure the property isn't Foreclosed?
(If so, you have chance to get it back IF it wasn't to long ago.)