bookworm252: reading your OP, my blood boiled on your behalf. IMO vile behaviour on the part of your hostess, and the friend's mother.
If I had been your boyfriend in the situation which you recount, I suspect that my actions would have been definitely not eHell-approved: envisage among others, having us packing up, and walking out of the hosts' house the following morning -- to spend our last few nights in the country in Salvation Army hostels or the equivalent, if funds allowed nothing better. magician5, as below, submits excellent etiquette-OK possibilities for handling the matter without lying down and letting oneself be trampled over -- which I doubt whether, in the situation, I'd have been able to think of.
At a certain point, you have to take back control:
1 - restate in clear but brief terms how the error happened, and "get a receipt" (get acknowledgement that you have been heard);
2 - restate your apology and any actions you may take to make it better;
3 - refuse to listen to any more of the harangue and leave the area.
A point to consider, made by a number of PPs, is that the two women may have had issues beyond the buying-for-underage-drinker incident, and taken those out on the OP -- with the incident, as a pretext. Trying hard to be more charitable and self-controlled than comes naturally at any rate to me: there are folk who are not outright bad people, but who can at times, under stress, behave like monsters. "Use every [person] after [their] deserts, and none would 'scape whipping", etc.
At all events, bookworm252 -- the way things played out, you were not at fault; the two women were very much so.