Have you called the local LP supplier to see if they will do a pump out? Or if the place that fills can empty it and you get a few $$ back?
I do not know if venting the Propane is an environmental issue, the smell is an additive, but the smell may linger after the tank is empty
If you do decide to vent it I would do so away from the house and only if it is breezy.
I know when there have been propane trucks that crash they quite often allow them to vent so there is no Chance of a big boom.