"wouldn't leading worship be leading? or singing a solo, encouraging? wouldn't both both demonstrate faith?"
Again, not to sound like I am targeting you, but, leading in church and singing do not demonstrate faith. They show personal belief. Faith is demonstrated when circumstances challenge your belief, and when Satan and the world attack a person and they do right anyway, trusting that obedience to God and His word is better than compromise.
Now faith is the substance of things hoped for and the evidence of things not seen.
...leading in church and singing do not demonstrate faith..
your probably right about that one. neither necessarily demonstrates faith.. unless you're leading/ singing in church in a hostile enviroment such as a non-christian country. but that's a whole different situation from megachurches.