Sensible home owners should enter a certain form of MOU/agreement or contract, and demand invoice + receipt from contractor. Every document is evidence. If deals turn bad you can always get help from Small Claims Tribunal (for claims not exceeding RM5000)

first you should document all defects, and retain all documents pertaining to asking him to repair the said defects, give a date, say within 14 days of the notice given, and when you receive no response, you can either 1) accept the defects if you deem it is acceptable;
2) should engage other contractor to rectify the defects using whatever amount you have left (from the final payment)
in both cases that contractor will not receive the final payment.
all these should be clearly stipulated in all written correspondence.

That headroom at staircase enough? Looks hazardous to hit your head. What do you need that 2 ft space for?