A: If you have not contested the Will and the Will has been admitted to probate, you are probably unable to require them to answer questions.
If you are contesting the Will then you are able to ask certain questions and get some discovery, relating to the Will contest. If the contest is successful then you might be able able to ask broader questions, but that depends on what happens after the successful contest.
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.