I read the 3 novels way long ago, and dont remember much of it now. But Aleal, Gandalf does not use his power lightly, he is one of the 5 wizards that have been around for thousands of years with the purpose of "balancing" and since they do it better than SoE dervs, they only use their power when is needed and to the point they dont turn the actions the other way. Nevertheless, I think its clear that he is the most powerful member of the fellowship.

And for the Sam thing, i think it comes on Tolkien's Unfinished Tales where he explain that Sam was actually a ring bearer. And at some point he has to leave with the elves too.

Lamont.