Consult with a lawyer. There are a number of reasons a trust (or an amendment) may not be valid, including incapacity of the person who created it, undue influence or fraud. The lawyer can obtain a copy of the trust and start from there.
There are statutes of limitation involved and once money is distributed it is hard to "unring the bell," so to speak, so don't delay.
This answer is intended to provide legal information, not legal advice. Legal advice should be provided by licensed attorney only after full disclosure of all facts. If you desire a no-obligation consultation to obtain legal advice, please contact me at 586-268-4463.
Not really sure what you are asking? From the vague facts it is clear you are sad about Dad being removed from the trust but I note you do not clarify what you mean when saying removed.....as a trustee? As a beneficiary? As both? Only theta trust agreement controls the process for removal and you would have to provide same to explore whether Dad was removed in a valid way in conformity with the terms of the trust agreement or something suspicious occurred.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.
The life insurance policy may have listed your dad as a beneficiary, so that passed outside the trust and was not affected by your grandfather's changes to exclude your father. If your grandfather was incompetent when he revised his trust to exclude your dad, a probate court might be willing to throw out that change. However, you need to act quickly. Once your aunt distributed/spends the money, it might be very difficult to get her to repay it.
I am sorry to hear of your family's loss and that you are dealing with this difficult situation. Your father needs to consult with an attorney immediately. There are ways to get a change in the trust thrown out, but they are not easy. For instance, your father would have to prove that his sister acted improperly, or that your grandfather was not able to make his own decisions because of his mental state.
The life insurance policy must have named your father as a beneficiary, which is why he still got it, regardless of what the trust says.
Feel free to have your father contact me for a free phone consultation.