Mexican licences are not valid anywhere in the US. Many nations have reciprocating licenses which are valid if the driver obtains a supplemental international drivers permit/license through AAA or other sanctioned entity. Mexico is not one of the countries that the US recognizes for licensing.
A US drivers license is honored in most European countries. All of the countries belonging to the European Union recognize the US drivers license.
According to the DMV & foreign visitors association, a US drivers license is mandatory regardless of whether or not an individual has an international drivers license for driving in the US. You may also double check that your employee had a valid international drivers license and not a forged or fake one.
to Mexico by land
Yes. All US drivers licenses are valid in all US states, and Puerto Rico is part of the US
Your birth certificate will not help you. You can enter the US from Mexico with an "enhanced" US drivers license, but only if you are traveling by sea or land, not by air.
A licence from Puerto Rico is a US licence, and is valid in all US states and territories.
Yes
A Pennsylvania driver's license is valid in all US states and territories. Most countries will allow a US driver's license holder to drive in their country, regardless of state.
No must have valid license or subject to impound
No passport required
Yes, seeing as Puerto Rico is part of the US