Ancient elf gives you a multiclass dedication feat, which is pretty slick, but I don't see anything exempting you from the dedication rule; are they just totally shut out from things like avenger or spellshot?
Yep. An elf who focused on a particular aspect to the point of having a class archetype would not represent being ancient by having dipped their toes into another class.
Ancient Elf is really good, but it does run into conflicts with things like class archetypes or free archetype rules. You can talk to your GM about making it even stronger than it already is, but usually the answer is just "don't take Ancient Elf with those options".