1) Seelah is using her Charisma SKILL, not her DIE - so, all Cha bonuses apply. The difference is also important because just changing the DIE (as opposed to the SKILL) would not affect the traits of the check - see below
2)Your question touches on perhaps the longest-standing and most hotly disputed issue with original PACG rules. Still, in the last several of months we've been given the official stance on what's intended when you "use one skill instead of another", though I can't be bothered to troll the threads for the specific post.
Where your particular issue is concerned, the official intent is:
- You're making an Intelligence check
- Seelah uses her power to use her Charisma skill *instead*
- the check thus becomes BOTH Intelligence AND Charisma check (i.e - it retains its original trait AND adds the skill used as a trait)
- Brilliance affects Intelligence checks and therefore applies
- Furthermore, you could ALSO play Eloquence (Brilliance's equivalent for Charisma checks) and it would apply too
Just confirming Longshot's post above. The current understanding is "If you use one skill in place of another, the original skill plus the skill you ended up using are both traits of the check".
You can see the official FAQ/errata here. The phrasing of the question might throw you off because it doesn't sound exactly like what you're asking, but the resolution should (hopefully) make it clear how it applies.