Can a registered will be contested if included in diff dispu

The query may be redrafted so as to convey a clear message.

Generically speaking, by limited understanding from the post:

If the partition and sub partitions are registered

And

If after registered partitions NO property is left in the hands of testator then since he is not owner he cannot dispose the property by WILL, since he does not own it.

If he still owns some property he can dispose the property by WILL.

Even registered WILL can be contested.

Registered WILL may not be easily set aside, merely, on the counts of authenticity.

If the matter is in court and WILL is put under question and notice is issued then put facts before court.

The testator originally recieved ancestral property parttioned though court. He sold a portion which the buyer had not settled and  thus a case was pending in court. In the meantime, after several years, he settled his property to his three legal heirs retaining a few portions for himself not mentioned in the settlement deed including the above portion under court dispute.  After two years, the testator registered a will of the property remaining in his name  to one of the legal heirs. Subsequently as a last wish he made a gift deed of the properties mentioned in the will to the same legal heir and a grandchild except for one property which had some complication in the registrar office.  This remained in the will. Subsequently after a couple of years the testator passed away while the case was still pending. To implead the legal heir into the court case, the gift deed, and the registered will were produced in court. The buyer/ plaintiff in order to drag the case has sought to include the other 2 legal heirs who have no say in the gifted property as well as the willed property, They seeing an opportunity are now trying to contest the will as well as the gift deed ( the testator's property after the  partition between the testator and his 3 legal heirs). The will does not exactly include ancestral property, but the gift deed includes a part of the testator's portion of  property.

1. As the testator has already done a partition accepted by his 3 legal heirs and kept for himself remaining properties one of which was in court, and   the testator exercised his right to gift and will his share of the property to one of the legal heirs who took care of the family  for more than 20 years . Can the other 2 legal heirs contest the gift deed and settlement and on what basis as they have already recieved their poriton while partittioning? 

2. Should they have been included in this court case in the first place as it appears to be  a ploy to delay the proceedings by the plaintifff who had not settled the testator's money?

3. What should be the course of action available for the beneficiary of the gift deed and the will to protect/ carry  out the last wishes of the testator?