If your thinking to use the file VIA the USB as in NOT transfering the file and just "pointing to it" instead I really don't know.
Probably not, as most apps expect the file to be where it's normally supposed to be, on the drive with the home folder with the rest of the game parts and may not "see" the alias or short cut because it doesn't recognize the external drive. All you can do is try, the worst that can happen is the mod won't work.
just play minecraft
no mods dont work
Mods come completed, there is nothing you have to finish to use them.
YOu download them.
Google for Modstallation. You can use that software to install mods.
You can if you use mods or server texturepacks
NOPE
Yes, you can download and install mods on the PC version of Minecraft, there are several gun mods you can apply.
You don't need a CD for mods. You don't need a CD for Minecraft at all.
the xbox 360 minecraft is made for xbox and can be found under games on the ps3 hardware on store. you need sufficient memory by either using an external hard drive, internal hard drive, or usb. you need to format the usb to use it under settings, memory.
A good way to get minecraft mods is to use Forge. With Forge, you can download mods of your choice instantly. Just download the Forge client, and you're good to go! :D
You should use forge.