I would also be concerned about the hole in the tile between the flue's. So he pushed his brush through two terra cotta flue liners to create this hole? Since the hole is between the flue's I am assuming it is above the damper of the fireplace where the Harman is located? I am not sure why it would be OK for the oil burner and not OK for a solid fuel unit. I would think the draft in the smaller flue could be affected by the hole to the larger flue whether it is oil or solid fuel creating the exhaust. Is the oil burner power vented?
Can you remover the damper plate in the larger flue to reach up to get access to patch the hole from that side?

