are you using two separate spells to handle the "True Seeing dispels invisibility but not etherealness? (e.g. one spl for immunities, the other for the 'illusionary protection' invisibility)
Not quite. I'm using a single SPL file with regular invisibility + protection from creature type [ANYONE]. The former can be removed by True Seeing, Detect Invisibility and the like but the latter can't. To the best of my knowledge, this is how it's supposed to work in PnP.
- which immunities are you granting? (I've used 'immunity to all weapons + 100% mr, all undispellable)
I use protection from creature type which makes it impossible to attack the creature physically and/or target it directly via spells. Area effect spells are still able to harm them, but only at half strength (50% resistance to all elements) except for force attacks (magical damage) which deal full damage to ethereal creatures. In general, aTweaks' Fiends usually retreat to the Ethereal Plane when seriously wounded in order let their natural regeneration do its job.
- are you allowing it to attack with non-physical abilities? (I've used apr set to 0 and 100% spell failure)
I don't allow ethereal creatures to attack anything on the Prime Material, neither physically nor via spells. In aTweaks, that's handled through scripting since the spell isn't available to the player.
- for cosmetic purposes I've used a simple "translucent effect", but your effect looks better imo...blur+translucent?
I used blur + translucent + color glow pulse + remove feet circle. Basically, I was trying get the unsettling, ghostly appearance which is described by the PnP source material. Looks like it worked.