1)As my brother has been the assessee in case of the house, plot and farm lands does the law empower him to undertake a construction, to lease or sell an asset either partly or whole without the consent of my sister and myself?
There is no law that can permit him to do so by him arbitrarily without taking yor consent or permission. If he does so, you may file an injunction suit as well as partition suit against him for relief and remedy.
2) Does he have the right to transfer the assets to his wife or children regardless of our consent?
Any such act without giving your due share shall be illegal, invalid and not binding on you.
3)Does the law have provisions that uphold the rights of married daughters for their appropriate share in their parents' properties in the absence of a will inspite of the lapse of time?
For seeking partition there is no time limit, you can seek t at any time or after a lapse of any amount of years.
4) If at all the district court rules in my favour can my brother challenge the verdict in a higher court and obtain a stay order?
Even if he does so, he cannot win because the law is very clear and in your favor.
5) What forums or grievance redressal platforms are available to offer counsel, guidance and material support in such case as mine?
No forum, only court of law.
6)As my sister and I signed the NOCs do we no longer be deemed legal heirs as the NOCs made on plain paper do not include relinquishing of rights by either of us?
Your fears are unnecessary. By natural law you both are also legal heirs to your parents, this NOC cannot deny you the rights.
7)Does my son qualify as a legal heir to his grandparents' property?
NO
8) Regarding the assets, the house and the site in town was acquired by our father at his own expense where as the farmlands passed on to our mother by virtue of her being a beneficiary of a will made by her maternal aunt
The properties are your parents absolute properties hence as a legal heir you are entitled to an equal share in the intestate properties. Dont worry about any other aspect.