not sure how you can solve this car rental problem
That's a real tough one - there are many hours of driving distance between Venice and Palermo - which being on the island of Sicily is almost like it was in another country! So I'm not surprised they would say no to a one-way minivan drop off there. They'll allow it for the smaller cars? Even with an extra fee they won't allow it? Will they allow one-way drop off elsewhere, like in the mainland?
I am not sure what to recommend, other than contacting some smaller car rental agencies that might be more willing to say yes, if you pay a supplement for the service. Hope someone else can chime in and offer some suggestions!