Waq,
Those are all very good and important questions.
But they are also very high level and complicated questions that really can't be properly answered here.
Please let me suggest a book that I recommend as a place to start.
"Welding Aluminum Theory and Practice" by the Aluminum Association.
I also suggest that while you study this, you hire an engineering consultant who is a welding specialist to help you with your current challenges.
http://www.browntechnical.org/products/welding-aluminum-theory-and-practice.html?pa=sc7125