Since your leaders won't back you up if you insist on X number of hours per parent I would send out a letter stating that if events do not have X numbers of volunteer in place by the dates specified that events will start being cancelled. Then cancel them. Remind parents when notifying them of the next event that the last event had to be cancelled due to their lack of volunteering and that it would be a shame for that to be repeated because it's sending extremely hurtful messages to the troop. If this doesn't work I would consider ending the troop. You get to the point where it's just not worth it. Parents can be an absolute nightmare to work with.
You might also want to consider adding something like, "if X number of events are cancelled due to lack of volunteers, there will be no more events planned for the rest of the year."
These suggestions right here are excellent advice. During your meeting, lay out very clearly what has been happening re: lack of assistance, what needs to happen to meet volunteer requirements, and what will happen if those needs are not met. Email this to everyone on your contact list, including all the parents.
I used to be a Girl Guide and I can remember at least one event being cancelled due to lack of volunteers. In fact, my troupe eventually disbanded because one of the leaders had to go on maternity leave and no one was willing to step up and cover her position (temporarily!) for the six months she would be absent.