The problem (as I see it) is with that unshown servo. Assuming that both servos are fixed in their location, as the aileron servo rotates the bar on the floor (which rotates the stick) it also moves the elevator servo arm slightly away from (or towards) the attachment point on the stick. Maybe this is just marginal and a bit of looseness on the elevator linkage would make up for it.

OK. Wait. I get it. By using the "collector pulleys" we're taking out the problem caused by the wide separation of the two connection points on the stick. Is this right?

No, the linkage is still a problem. I think.

BTW, I picked up a couple of these the other day: