Haven't read everything yet, but so far I'd mention this. A Dominator shouldn't automatically control all slivers in range. Instead it should have the ability to claim all unclaimed slivers in range as an action, an control claimed slivers. The exception being if the sliver is already claimed by another Dominator, in which case it must claim it individually (taking an action), and it is then a contest of wills to control it would be had.
Possibly also allow the Dominator to prep defence for some number of slivers each round, granting it a bonus to maintain control over those ones if another Dominator tries to take it (with prepping the same one multiple times stacking, and possibly allowing it to forgo prepping slivers to gain a bonus on taking one, albeit a smaller one).
Mind that whole part would likely be rather in games since players aren't likely to play a Dominator, but if they did.
Owrtho