WALL OF TEXT INCOMING (but I think I fixed the Deceiver + ethereal super hp bug).
WALL OF TEXT
The bug where all units in the game bar the Tau player who's controlling the Ethereal is caused by: the modifier "enable_attack_allies" used by the Deciever's ranged attack "necron_deceiver_blast_nodmg_confusion.rgd" interacting with any modifier that affects a player globally. In this case, Ethereal's "tau_for_the_greater_good.rgd" ability. When the enable_attack_allies gets applied to the Ethereal, the greater good ability's modifiers 1 and 3, using the apply types "modifiers\health_maximum_player_modifier" and "modifiers\morale_maximum_player_modifier.lua" seem to get reversely applied to all players but the original owner, permanently too for some reason. The result however is a permanent +800 hp for all units but the Tau's every time the modifiers interact.
The solution to this seems to be to avoid modifiers using the application types that affect the player directly and instead use modifiers that apply to unit types instead. This means that you'll have to remove all modifiers bar modifier_02 on the greater good ability and replace them with modifiers that give the additional 200 hp and morale to the specific Tau unit types instead. The problem with this is that there might not be enough modifier slots to fit in the boni for all the required Tau units.
I've only ran a quick lab on this to test if it works but it seemed that at least once removing the global player boni, applying the enable_attack_allies modifier to the Ethereal no longer cause the sudden surge of hundreds of extra hp to the opposing units.