Yes.With NSMB4 Level Editor
There is no Wario, unless you edit the graphics.
Open a Super Mario World ROM with it. You can now edit the levels!
You can't.
No, you cannot play as Bowser in the new "Super Mario Brothers" game.
I think there's a code on the internet for the Action Replay where you can edit your own levels. If there is then download that code, then copy it onto your Action Replay.
You become able to edit Super Mario Galaxy games. But for that, Whitehole is better.
Mario Sports Mix for Wii. but hopefully users may edit this because Nintendo might be making more Mario games in 2011
you cant without an action replay, or you could download a ROM or (reccomended) download toads tool 64 and a super Mario 64 ROM extender to edit the texture, its really cool
You Can't, but after you beat the game, you can find Yoshi eggs in all the levels. EDIT: After painstakingly getting all Yoshi Eggs, you can see a Yoshi on the title screen, and even though people say he's playable, he's not...
In order to get to peach you have to complete the game. This usually requires 70 stars, although some people have gotten 120 stars before they go into the final level.
At this point in time you cannot edit levels with friends.
No.