To understand the full story behind its legendary status, see our guide on why Black Lotus is so expensive
I don't think it's necessarily the best way to use Toph, but it is straightforward and effective
Other players can choose the Battle as a target for any number of their attacking creatures, whilst the protector player may choose to defend it by blocking
If the copied permanent is a token, the token created with Urza copies the original characteristics of that token as stated by the effect that created that token, with the listed exceptions
If you can manipulate the top of your library to make sure that the right creatures are there, you can guarantee the lines of play in your deck will work smoothly
Theres no activated ability to sink the mana into, and planeswalkers can only activate their loyalty abilities once per turn, so I forgive you if its not obvious it was lost on me when I first looked at the card