sometimes
Not in the current Vanilla (Minecraft without Mods) Minecraft, no. But their are a few Mods out there that will make that possible.
you can get minecraft mods and textures off the minecraft mods website.
No it might be that you have an out dated mod that crashes to game or that you do not have something like modloader (which you need for most mods). Also when downloading mods make sure they are for your current version of minecraft or it will defiantly crash.
Yes, some may affect the game and its performance. If you have a slow computer, I don't recommend that you download too many mods.
bearshare is not required to get minecraft mods.
there are no mods in minecraft pe i hope there will be
Yes, most Minecraft Mods are updatable.
you can get most minecraft mods off the minecraft mods website, or you can google.
Unplug your hard drive and put it in your computer and get mods for minecraft
On Minecraft Forums
you can easily get mods from planetminecraft
Minecraft Forge lists you all mods in minecraft and Forge