My understanding is slightly different from what you have said. The prerequisite for Agonizing Blast is the Eldrich Blast cantrip, which you have through the Spell Sniper feat. In essence you are a warlock since you can use Eldritch Blast and, as such, you meet the prerequisite for Eldritch Adept. Does it make sense?
I think the wording in the Eldritch Adept feat is there so that you can't use Invocations that require 7th level warlock or Pact of the Tome, etc. However, there are plenty of other Invocations that could be used, such as Eldritch Sight, Fiendish Vigor and Mask of Many Faces, for example.