Invisibility is a Polymorph (as of right now, might be subject to change) spell and it comes in scrolls and potions too. None require Source. Doesn't sound implausible to me that a kid or Griff's men would have one of those.

I agree on Source skills in general though and Walk in Shadows in particular - it's garbage to the point where it's not worth using, especially compared to the silly overpowered Source skills. Steam Lance sounds really underwhelming too, but I'm guessing skill crafting might improve it, depending on how it's implemented.
Honestly, 3 slots requirement is an overkill for most Source skills, that's something Larian will be able to balance rather easily.

And I'm not touching the NPC stat issue with a ten foot pole.