I doubt that a partial release of the loan would happen. However, your $115,000 would be enough to pay off the $70,000 balance. The $115,000 would be put in escrow. During the escrow period the subdividing would occur, your part of the property would be deeded to you at close of escrow and the mortgage company paid. The other owners would just continue owning the remaining property.
You'll probably need a survey. Maybe a thousand or two. Call surveyors to find out. Subdivision fees. Likely county level. Call and find out. Escrow company fees, title insurance. Call title company and find out.
Then why not buy the whole property for the $115,000? Would seem a lot less complicated.