You can use it to operate the elevator in the rocket hideout, under the casino in celadon city. Hope this helped you!
yes
Use the pokeflute.
Use a gameshark
If there in the way of your path then use the pokeflute.
Yes you can trade leafgreen version Pokemon to another leafgreen version.
use a code
use moon stone
Yes you can.
use thunderstone
Use Cut on the door.
Yes, or a wireless transmitter.
nothing use it as a Pokemon nothing to do with any events