RE: Manipulating aileron linkages for perfect rolls?
A large part of achieving truly axial rolls is to ensure that the CG is located back very close to or right on the neutral point of the model and that the side area of the fuselage and fin is distributed with a 25-75% split on either side of the CG as well.
Much of what you are concerned with is related to the normal setting of the CG ahead of the NP in order to have some pitch stability. However that same stability results in a strong pitching towards the upper side of the model when it's not actually flying upright.
A secondary but still important point is to ensure that the major drag producing elements all have their centers of drag located so that the overall drag center of the model is as close to the true 3 dimensional CG (as opposed to the balance point) as possible.
Do all this and you end up with something that looks remarkably like a modern pattern model. Or in the case of a biplane much like Chip Hydes show bipe. From there it's just a matter of playing with the control surface throws to ensure you get even and linear throws just like you're doing.