There's no evidence that the accused was actually driving on the wrong side of the road. This is just a typical american stereotype. There have been numerous accidents along this road between british drivers. (Note; there's a very dangerous hairpin turn).

Also, the accusation that she is directly responsible for the fatality cannot be supported, so any legal proceedings would be purely for financial gain/reputation. It is likely that she was extended diplomatic immunity by the US state department to avoid this unnecessary public scrutiny/legal battle.

I think the family's attorney and the UK government should just disclose these facts so the family can move on and grieve in private.