If you friend has a guardian, he has been declared incapacitated and has no ability to sign a power of attorney. In any event, to change a guardianship, you must file an appropriate application with the Court which created the guardianship to change or terminate the guardianship. If your friend wants you to be his guardian, the Court will consider his desire in making its decision.
If the will created a trust for your children, and it should, then the beneficiary should be 1) primary = spouse and 2) secondary = Trustee of Testamentary Trust per last will and testament. If you have ANY doubt what you are doing, consult the attorney who drafted the will. If an attorney didn't draft the will, hire one and be prepared to shred what you have.
Muniment of Title does not get you letters testamentary, and from what you describe, I can't tell where you are in the process and what needs to be done. If the lawyer is not available after paying $15000, then find another lawyer. You need a lawyer's guidance. This is simply too complex to answer in this type of forum without a face-to-face interview.
I think the language you quote is pretty clear. The car identified doesn't exist, so that gift lapses. You don't substitute another car for the one described. The car that does exist, unless gifted by another bequest, becomes part of the residuary estate and goes to the person identified in the second clause that you quoted.
This could have been clarified by probate, which I gather didn't occur. Regardless, you possess the item, they claim it belonged to your mother, that it then should be theirs (I don't know why). But, it is too old to open an estate, and you aren't legally or ethically obligated to turn it over to them. You do not need to surrender it to them and they can not get a court order to make you do so.
Well, happy Thanksgiving!? Sorry these issues burden you today and I apologize for this necessarily vague answer. The issues here depend on the ownership of the property (was is community or separate) and the terms do the deceased husband's will. The will may purport to give to the son property belonging to the widow, so the will may not be what it seems. But any decent answer requires an understanding of the assets and examination of the will at issue. Please have your mother in law...
Any can file, but all three should be appointed unless they waive their right to serve. There is no priority among the three, by age or otherwise, unless the will gives such a priority. Equal executors means what is says: it takes all three.