Happy New Year to you. Sorry to hear about the problem. Surprised to hear what you have gone through as Mah Sing is a big developer. Not staying there, I won't know about it. 

To answer your questions as I have experience on that :

1. As long as you have lodged the defects officially and that the develop has acknowledged it in writing (all black and white) before the expiration period, they have to complete it. It is only the new defects that are not covered. So, you must get all the defects lodged. Secondly, you must ensure that they acknowledge the submission and everything is black and white and not verbal. Verbal does not help at all.

2. Construction during off hours. This is termed as "nuisance". It is the management's duty to ensure that it does not happen. If it happens, you can lodge a report or complaint to MBPP's Department of Building.

Hope that helps

Threaten to take the management to the MBPP is usually enough to scare them away