3) may not be neat but it will work:
Create a trigger. not sure how to activate it on action, but to activate it by radio, select, "Radio Alpha" for activation.
Call the trigger x
In the on activation field put:
MG="m2"camcreate[getpos x select 0, getpos x select 1,0]
MG=name you give to this unit
"m2"=thing you are creating...i thinks its m2, but you may have to play around
The 0 at the end is height.
Also, under "Effects" you may want to do "fade in" (its in the bottom box of the effects page, i think) this will make it so the machine gun doesnt just appear, but will be there after it fades back in.
Thats as best i can do
Someone else better fill in what i cant do, like trigger it through the action menu thing ;D