That is pretty close to how we ruled it, though the 30cm range part we left out (since it was not in my version of BFG:R Dark Eldar - from afterimagedan's site). Here's what we said:
[spoiler]Vs. Enemy Ships
Enemy ships may not shoot at, board, or otherwise attack
a mimicking vessel. In addition, they may not launch
torpedoes at such vessels unless the path of the marker
brings it in contact with a non-mimicking enemy target.
Vs. Enemy Ordnance
Mimicking ships are ignored by enemy minefields and
will not attract mines. However, contact with any enemy
ordnance is still resolved normally. If enemy ordnance
markers attack a mimicking ship (or any ordnance the
ship has launched), that ship is automatically revealed.
Revealed
If a mimicking ship fires its weapons, launches torpedoes
(or mines), uses its own ordnance against an enemy
ship or marker, or otherwise attacks in any way, it is
immediately revealed.[/spoiler]