Counterpoint: The main character still going around with a small party is still believable because they are still the only one capable of sealing the numerous rifts along the countryside. Plus they run into numerous enemies who get completely caught off guard by the Inquisitor's party banging on their front door. They're not doing that while holed up in Haven or Skyhold, and coming with an actual army would have basically just told all the enemy forces where the Inquisitor was (and where they WEREN'T), and what they were planning to do.
The main character going around with the small party is the part that doesn't make sense. What you describe - a small force doing surprise missions - is the job you give your agents, scouts, spies or soldiers. It's not the job you give to the only character who is capable of closing the magical portal in the sky, because if they die, you lose. Yet no one cares if the inquisitor elects to go on a solo adventure travelling around the countryside.
