The query “Can I take advantage of spell slots to solid the spell granted from the Fey Teleportation feat extra instances?” requested whether or not a personality can use a spell slot to solid a spell “discovered” via a feat equivalent to Magic Provoke or Fey Teleportation. The reply, in a nutshell, was: Sure, however provided that the spell is on the spell record of the category offering the slot. So, for instance, an elven wizard who learns misty step by taking the Fey Teleportation feat may use wizard spell slots to solid misty step, as a result of misty step is on the wizard spell record.
A bit of the answer is rationale was the next steering from Sage Recommendation Compendium v2.2, p. 8 (emphasis mine):
In case you have spell slots, can you employ them to solid the 1st-level spell you study with the Magic Provoke feat?
Sure, however provided that the category you decide for the feat is certainly one of your lessons…. Briefly, you have to observe your character’s regular spellcasting guidelines, which decide whether or not you may expend spell slots on the 1st-level spell you study from Magic Provoke.
We could say a multiclassed Rogue 3 (Arcane Trickster)/Wizard 2. Based on the spellcasting guidelines for multiclassed characters, see PHB p. 164-165, this character would have two 2nd-level spell slots however wouldn’t — at the very least not via his class ranges — have the ability to study any 2nd-level spells.
Now think about that this character positive aspects a stage, turns into a Rogue 4/Wizard 2, and takes the Fey Teleportation feat. He “learns” misty step, which is on each the the wizard spell record and the Arcane Trickster spell record (as a result of the latter is admittedly the previous). He nonetheless has no entry to 2nd-level spells via his ranges in both class.
Can this character use spell slots to solid misty step though he could not in any other case solid, and even study, any 2nd-level spells?