In the normal situation the sale of property by executing a registered sale deed is considered to be complete. In your case you have not expressed your willingness to purchase the neighbor's plot before he looked for a prospective buyer outside, henc you may have lost your preemption rights in this regard.
About the ancestral property, under the Hindu Succession Act, co-owners of ancestral undivided property typically have pre-emption rights.
This means if one co-owner wishes to sell their share, they are required to first offer it to the other co-owners before selling it to an outsider.
The purpose of pre-emption rights in ancestral property is to give the co-owners the first option to buy, the property is less likely to be split up among multiple owners, which can lead to disputes and inefficiencies.
It ensures that the co-owners have a chance to maintain their share of the property and avoid unwanted ownership from strangers.
When a co-owner decides to sell their share, they must first offer it to the other co-owners at the same price.
If the other co-owners refuse to exercise their pre-emption right, the seller is then free to sell the share to an outsider.
Therefore there is no danger about the denial of preemption rights in filing a suit for partition.
You cannot compel other cosharers to transfer their respective share in the ancestral property into a trust for any reason, if one of them is not agreeing then you may have to leave him and his share alone while forming the proposed trust, then also your purpose will be defeated.
In cases of disputes or disagreements, the courts may intervene to resolve the matter and ensure that the pre-emption rights are properly exercised.