A: If they are beneficiaries, then that can create a problem for each, the father more so then for the son.
If there are at least two disinterested witnesses to the Will then both father and son would receive the benefits under the Will.
Otherwise, while the Will might be valid, any disposition to the father and son or appointment as executor or trustee would be void.
In addition, if the father-draftsman was named a beneficiary under the Will, he might be in violation of conflict of interest rules if another lawyer would conclude that the transaction was not fair and reasonable.
A: To best answer your question one would have to read your father's Will. For example, if your father's Will created a trust for your father's wife funded by all his assets with her getting the income for life and on her death, the assets pass to you and your siblings, then it is possible that your father's Will gives her a power of appointment to change the direction of the assets by including a provision for that in her Will. It is also possible that the job settlement passed to your mother outside your father's estate, in which case, she can direct where that goes on her death.
If you are named as the executor of your father's estate in his Will, then you have go to court to have the Will admitted to probate to be formally appointed. As executor you have the power to collect and his assets and disburse them to your father's wife and the four of his children as provided in his Will. If you suspect your changes were made to his Will before probate then you could object to the changed Will being admitted to probate.
While your father's wife cannot make changes to you father's Will (the exercise of a power of appointment is not a change to your father's Will) she can make her own Will to dispose her assets on her death, unless she and your father had reciprocal Wills, which in most cases cannot be changed without the consent of the other spouse. If your question is limited to your mother making a Will leaving her interest in your father's estate to her adopted daughter and boyfriend, then she is within her rights to do so, assuming there were no reciprocal Wills.
A: Because your grandfather left a Will, it has to be probated by the court if he had assets that require probate. You as the grandson have no direct stake in the estate.
As you describe it, your mother would have to be named in the section of the probate petition naming all people with an interest in the estate, even if the Will does not name her as a beneficiary. For the probate to proceed, a Notice of Probate has to be served upon your mother if she does not consent to the petition and the person filing the petition would then have to get a date from the court for a review. Is she does not consent, your mother would have be served with what is call a Citation advising her of the court date. A copy of the Will has to be included with the Citation. Then your mother would have a chance to read the Will and decide if she wants to object to the Will being admitted into probate. If she does not consent and does not appear, whether in person or by an attorney, on the date set by the court in the Citation, the court is likely to approve the petition, which serves to validate the Will.
If you are able to get a copy of the Will from your grandfather's wife your mother can then decide what to do much sooner.
Because your mother lives in the Caribbean, service on her can be done very easily but in a way that she might not actually receive at all or in time for the court date.
Under those circumstances it might be best for your mother to contact a lawyer who can search the court files to see if a petition was filed without waiting to receive the Notice of Probate or Citation.