We had a similar scenario of US expat friend visiting the US every summer from her home in the UK.
For the temporary medical coverages while traveling in the US, we found Generali Travel Insurance (used to be CSA) has the most extensive medical and emergency medical transport coverages--even on the Standard plan so cost of the premium is decent.
RE: rental car insurance, all plans cover rental car damage but not sure if the other driver/car is covered if you are @ fault. Their phone is on
www.generalitravelinsurance.com to confirm with the experts.