While I am no sour beer expert I will try to answer a couple of these to the best of my knowledge.
1. I would repitch some yeast at bottling time (based on http://www.homebrewtalk.com/f72/drunk-owl-mango-55227/
2. I would say you could just use a gallon to blend if you're trying to create a Geuze type beer; but you might be able to "yeast harvest" the dregs I'm sure they will have enough bugs if that's what you're shooting for.
3. I don't know.
4. Sorry I don't know, I just started my first sour beer a few day ago.
Can I get a recipe for this beer? I love Rodenbach!