You could analyze this series until the cows come home, clearly my favorite series of all time, and a great graphic novel achievement. I agree with a lot of what you say, except for the fact that you didn't LOVE Risso's art. Art's subjective, but Risso was a HUGE part of the series and no other artist could have brought to the table what he did. He's a co-creator and anytime you saw a story behind the story, it was him fitting that in. There's few artists who could come close to what he did in these 100 issues.
The Painting is a metaphor for art. In the end, it was meaningless, much like most high priced art. Worthless or valuable to only those who put value in it. Don't think too hard on that one.
As for your other questions, answers are there and you may see it more clearly on the second time around knowing how everything fits. Everything is there, trust me. But you gotta love a series you WANT to read multiple times because it's that enjoyable. Too often I buy comics that I read once and never want to read again, or worse yet, share with anyone. 100 Bullets, is always in an arms reach and has been loaned/bought to friends/family numerous times.