The length below the sink connection is immaterial. A cleanout in the attic would be useless. You DO need a cleanout below the sink connection's "T", and that could be used to clear any obstruction in the overhead vent. It is up to the authorities whether that offset would be acceptable, but there is no reason it should not be in this case. I have even used an "island vent" setup with the foot vent connected back to the vent through the roof.