inheritance of Property under Hindu law .

Only the legal heirs of the grandfather have rights on it and no issues on separation or living jointly

Agreed with the views expressed by the above experts.

A child in womb also gets a share in HUF, irrespective of the fact his/her parents live separate (with or without divorce).

It depends upon various facts also.

1. Whether Great Grandfather died intestate?
2. If died intestate, how many legal heirs were there, and in the total assets how much your Grandfather could get
3. Whether your Grandfather is alive?
4. How many legal heirs your grandfather had? and how much your father got to his share?
5. Now coming to your father's share, it is possible you can claim your part depending upon other heirs.

In the above sequence, whether at any stage the property was shared by way of Registered Sale Deed in the absence of a Will?

If the parents secured/eligible for share, the proportionate share devolves on their legal heirs together with other co-sharers of his parents.  Separation of parents is not going to make any difference unless there are agreements to the contrary giving some benefits at the time of separation in lieu of the shares in family properties.

Recently, the Supreme Court (SC) reiterated that all assets belonging to an HUF would be presumed to be joint property. Under the Indian law, the property belongs to everyone in the family, equally. 

Children's rights in their father's ancestral property are not affected upon divorce. Unless there is a will excluding them from inheriting the ancestral property. 

The son is treated as a Class I heir of his father's property. He has a legal right over his father's ancestral property. He also has an equal share in his father's self-acquired property if the father dies intestate.  

According to the Mitakshara School under Hindu Law, the son has a right by birth in his father's and grandfather's property. If it is a self-acquired property of the parents/father, the son cannot claim it. But there can be a consideration regarding the same if he can prove his contribution to the property.  The self-acquired property is unlike ancestral property.

A son is his father's legal heir and coparcener in ancestral property. If the parents get divorced, a son gets his share of inheritance in the ancestral property, as it is his birthright

Is the great grandfather alive now?  Who is in possession/occupation of the building? What is the source of his/her title to property? 

minor has share in the share of his father.  If he wants his share he can file a suit for partition against his father thru., minor guardian.