ORIGINAL: MegaByte-2
From reading your post it occured to me that the counter-rotating wash across the tail may be a problem pushing turbulated air across/around it rendering it less effective.
You're right and the problem is finding that sweet spot where everything
works in unison instead of working against each other. The closer the V
comes to parallel the more wobble the plane develops and the harder it
is to control.
The diagrams I made up shows a way to adjust the V tail, or any other
tail, so that you can find that spot without constantly cutting & gluing.
It also makes it easier to adjust for different modes of flying without
adjusting rudder and stab flaps.