pre-nuptial agreements?
am no loyar but a layman's thinking to these issues:

Point No 2 : If one party refuse to serve the instalment, what are the actions/ remedies available to counter such situation?
perhaps you can word in the agreement that in the event one of the parties failed to serve the installment or late by x no. of days, the other party would automatically be deemed to have waived their interest on the property and hence the other party would have the first right of refusal to purchase from the defaulting party at market valuation rate.

Point No 4(b) If either party disagree with the selling price for selling to third party, what are the solutions available?
that is the beauty of agreements. you can't disagree. but to be fair, you may word it that the selling price would be based on average valuation of 3 reputable valuation firms versus the offer price. and the offer price shall not deviate from the average valuation price by more than 20%.

cheers!