I think that if you can get iso training into your routine, you are better off.
To answer your questions:
1. Pros - It helps work out imbalances, and also is more realistic for real life.
Cons - You may not get the same weight due to grip issues. This is a problem with BB and DB.
2. I can triple about 60% with a DB in front, and about 50% to the side. But I get more stability work to the side. I would start about 40% and see how that works.
3. I use DB in front and BB to the side. Keeping the BB balanced is just as tough as lifting the weight. Takes a little practice.
4. Both. They both have uses. Use the one that you like best.
6. Starting with DB is easier, except that you don't have the alternate grip. There is no resistance to roll. When you start out, your grip may be the limiting factor. Just get used to it. Don't use straps. You won't get stronger that way. Use chalk.
Balancing a BB with one hand is fun. You will want to start really light with the BB. Otherwise, you can look at several handles available, and stack your weight on a pin. This gives you much more flexibility in your technique. Static handles, rolling handles, offset dumbells and full length barbells all have their own challenges. Also, DB's make you start much lower to the ground than BB. This changes the lift as well.