Hi,
From my experience, the maximum length of stay that the Customs Border Patrol (CBP) at the Port of Entry (POE) grants is 6 months. Please note that this is not guaranteed and may depend on the assessment of the CBP still, as they may grant shorter depending on the purpose of the visit of the traveler.
The PH immigration normally asks for the airline ticket which is ideally should be roundtrip for visitors, if not, your Mother in law should be prepared to respond to the inquiry why the single trip. For the travel insurance requirement, I cant seem to find an official source. It may be worth buying one just in case.
https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html
https://ph.usembassy.gov/covid-19-and-travel-information/