Can a sheep decide it is ready to put on a suit and tie...and step up authorized to lead the other sheep in the flock? How is that going to turn out in the long term?
From my experience, it is almost impossible to find anything dressy enough for sheep. Their sizes are confusing, also. A suit should "drape" nicely and the four feet of your typical sheep don't lend themselves to the cut of your regular business suit. So, my answer, to the first part of your question, is "no." You have the question of a fashionable tie to go with it also and they tend to drag the ground as the sheep run.
As far as stepping up to do any type of leading, I'd conjecture that, too, would be impossible because sheep tend to be extraordinarily stupid and would have trouble formulating a cogent thought as to the leadership role. Uncertainty and dischord would follow and chaos would surely ensue.
When the Shepherd returns...will he then follow the opinions and rules of the sheep in the suit and tie?
Again, from my experiences in the sheep haberdashery arena, I'd say that even if you find the appropriate suit and tie, the sheep would still be dumb as shit and so the shepherd (unless this person is a functional idiot) will laugh at the sight of sheep dressed like Wall St. bankers and probably hurt himself. He will also wonder just who was able to find the fashionable attire for the sheep and/or who was stupid enough to dress the damn things up like something from Orwell's 1984. He won't follow anything the sheep may say.
I hope this answered your sheep - fashion - leadership queries.