knee wall no venting possible
I'm having trouble visualizing the whole layout. That said I also have knee walls and sloped ceilings. I was able to force attic baffle vents between the insulation and roof by taping them together, then pushing them up with 1x2 sticks. It's not perfect I'm sure, but does allow some ventilation from behind the kneewall to the main attic. Of course, I have soffit vents at the top plate behind the kneewall and a standard ridge vents at the top of the roof.

If you really can't add soffit or ridge vents maybe consider making it a hot roof and add more insulation on the actual slope ceiling/roof, either interior or exterior (if a new roof is in order anytime soon)