hmmmm lemme try:
1. i dont think is possible
2. could be a spawned unit? (BTW Selectable=no would be a good start

)
3. what he sed ---^
4. you could just make a unit that can walk and fly (depending on distance of path making decision) again what he sed ----^
5. you could do a unit (that you'd have to put on a map not on there in the first place) with a virtualscanner with a wide range (depending how far it will search for units) and a weapon of whatever you want. That could also be used for #2, xcept i dunno if u want them to be trained from a player or whatever
Edit: (this method is kinda like the alligator but they only attack things right next to em)
6. i dont think its possible for RA2, it probably is in YR tho
