Why your son is not interested to come with you is he greedy of his father's money?
It seems your son is also not a good son.
Why you waited for 20 years in hell if your husband was not good(because you want to divorce hime)?
If my above assumptions are wrong you are going to loose in place of gain.
A) What can I expect from my husband’s side as part of settlement ( Just 50% of the flat in Mumbai or more)
Ans: You can not get 50% as it not a rule as of now wait for some time so that parliament pass that bill
You can get residential rights.
B) How can I get more and what maximum can I ask for?
Ans: If your are not well qualified then on the basis of your husband earning you can get good maintenance amoumnt (that can be one time also)
C) What would be procedure for sale , if any?
Ans: If you are not the owner where is the question of sale?