You keep using that word. I do not think it means what you think it means.
Ahhh....my misspent youth...
1. No. If they start in concealment terrain they can start with a conceal marker.
2. It really can't. Here is an excerpt from the rules:
Vehicles and Guns may be covered with a Conceal marker and
have the marker removed in the same way as an infantry unit.
In addition, a Vehicle has its Conceal marker removed as soon
as it is in the LOS of an enemy unit, even if it is in concealment
terrain. Conceal markers are not removed until both sides have
completed set up.
3. Sure. I had not thought of it in those terms, but when you see a decoy/concealment marker, you roll an attack, compare it to the infantry firepower, and remove the conceal/decoy if you get a suppression result.
4. Obviously, any movement of a real unit is governed by the rules of the game. You lose the benefit of a decoy if you make it act outside of those rules.
5. Okay, I think I see where this is going now. If you go to the extreme you could pick up a decoy and put it anywhere and then shuffle the counters. The player would know one of the counters was a decoy, but would not know which one....no, that is not my intention. Any decoy needs to act within the limits of any units you have in the scenario.