Before you get any further with this plan, see what it will cost to pick up a car in Germany and drop it in Italy. On this Forum, the surcharge for a cross-border drop off has been posted as ranging from €50 to €600 (yes, six hundred euros). If the fees work for you, great; if they don't, you'll have to come up with a different strategy.
Next, where in Germany is the wedding? You may find it more convenient, as well as cheaper, to fly from Germany to Venice or Treviso airport, then rent a car at the airport to get to Pordenone.
For a day trip to Venice, it will probably be easier to take a train, since you can't take a car any further into Venice than Piazzale Roma, and parking there is expensive.
I haven't been to Fruili-Venezia-Giulia, but my friend went to Udine and enjoyed it a lot. This was in September, and his visit coincided with a food festival in Udine. Googling, I see it's September 7-10, 2017, if you can make this work with your other dates.