I have an idea that I think would make taking Shadowdancer levels possibly more appealing for the Shadow Mage class.Shadow (Weave) Mage is a path for Wizard and Sorcerer characters.
A Shadow Mage uses the Shadow Weave to power their spells. This means:
They cannot cast Evocation spells (except Darkness, and including Hellball and Greater Ruin).
They cannot become Wild Mages, and are not affected by Wild Magic zones.
They cannot cast arcane spells if their deity is not Shar.
May take levels in Shadowdancer without meeting the usual skill and feat requirements. Instead of gaining Sneak Attack from their shadow, Shadow Mages get +1 caster level per Shadowdancer level.
Shadow Mages get +2 DC on all Illusion, Enchantment and Necromancy spells.
At Wizard level 20, they are granted the Hide in Plain Sight feat.
(This means that multiclassing into the Shadowdancer class is no longer necessary for HiPS; however, you may still do so should you wish to benefit from the Shadowdancer's other abilities.)
There is no reason to take 5 levels of Shadowdancer for HIPS. So if you are taking Shadowdancer levels with the Shadow Mage class I would think most people would want to put less than 20 levels of wizard, and be a sub-par caster with high defense against dispel, but no epic feats. That's what the +1 to caster level with the Shadowdancer's summon is for right?
As a caster class you want epic wizard feats. Especially when the build focuses around 3 cool schools of magic with nice epic spell focus features. For my Shadow Mage I find it necessary to take ESF: Illusion. The other two are just nice bonuses.
As a melee or ranged Shadowdancer you want high damage, so often times people go for rogue, assassin, blackguard, or monk classes to add to Shadowdancer's damage. Which seems to me why you fixed the Shadowdancer's original summon with the added sneak attack damage so that people could put more levels into Shadowdancer without losing too much damage. As a Shadow Mage you lose that and yet you would also lose epic spell focus.
I think the idea for giving the Shadow Mage/Shadowdancer +1 caster level is cool, but it doesn't work like the Arcane Trickster class (not included in NWN1). So it only applies to your DC's against things like dispel, which is good for a sub-par caster/SD build. But then you have a build that's attacks and spell casting seem to be both not so great.
So what I propose to fix the build would be to allow epic Shadowdancers that are Shadow Mage builds take Epic Spell Focus in the 3 schools of magic that Shadow Mage's get bonus's in (Illusion, Enchanting, or Necromancy).
That way they can still get a decent epic shadow lord that helps against dispels and epic dodge, but also can use the ESF: Illusion, Necromancy, or Enchanting features you guys added that are so cool that I feel most people would choose to take over putting any levels into Shadowdancer as a Shadow Mage at all. Maybe ban mummy dust from the epic feat list so that they concentrate more on the shadow lord summon and shades spell. ESF: Illusion and Enchanting suits the build better anyways.
(Edit: I'd like to add that if people put more levels into SD and less into wizard to get more epic SD&SM feats, they would not be an overpowered caster, but just a decent SD caster that has lower melee or ranged attacks, but decent summons and options to quicken cast spells, but not an overpowered auto quick cast set SD build.)
For now I am building 30 wizard, Shadow Mage to avoid losing cool epic feats. But the struggle of having no epic shadow lord is real lol...