Depends on how you define reasonable. If you believe that the independence of millions of subjects in the european colonies, and international free trade for all nations(where people from say, india can buy european goods other than british ones etc) was a reasonable cause, then yes the real causes of ww2 were reasonable.And the reasons for the World Wars we had in real life were reasonable?
Exactly. But more importantly, whenever he carried out his plan he would have to become the juubi jin, which he has shown he can become now.The OP has a point though. He should have summoned the incomplete Juubi and merged with it to become Juubito before the war even started. He could have solo'd most of the war by himself then.
The villages would never have agreed.Again, you're taking a shortcut to the Master Plan, which is why Infinite Tsukiyomi is not up and running right now. If you're going to settle for the easy way out, then there's bound to be a flaw. There's a big chance that Obito has a flaw right now that has yet to be exploited. He had to deviate from the original plan, which is why all this is happening in the first place. Plans don't always work out the way they want.
Also, its a story. How many stories do you know take the shortest route to a goal with no problems?
If Tobi wanted to take it the easy way, he could have just captured the 5 Kages and force the villages to hand over Naruto & Bee.
You also miss the point. Obito himself stated that for the purpose of the moons eye plan , an incomplete juubi would suffice. Meaning all the action in the war he started and carried on was pointless. Also, his unstable state atm was gonna happen sooner or later. If he had revived the full juubi, with the 8 and 9 tails controlling it as its jin wouldve been even more troublesome.