Dennis,
Cool project. Some random thoughts:
On a good airplane design, there is usually a type of hardware and installation that was intended to go into it. The trick is to see how the original was done, or a successful copy was done, and get your hands on that stuff.... I assume from your pictures that the buttons are fixed, not free to rotate. True? If so, then you need to use double loops on your lines.
If I planned to eventually fly the beast, I can't say I'd be happy about building the lines into the plane permanently--or even semi-permanently. There has got to be a way to get at it. If the wing sits above the pan, then the buttons would best face down.
What are you going to close out the grooves in the wing with? If you used 1/64" or even 1/32" plywood strips you could have the buttons pretty close to the bottom of the wing. Again, this works best if the buttons face down. So in your mockup, it would seem that you could get it centered ok, and the only issue would be whether you are going to be happy with the thickness of the upper and especially the lower bellcrank support.
A hole size of .135" seems a bit odd to me. Who has got rod stock that size laying around? It's not even a close fit for 9/64", which would be .140. It doesn't look metric, either. But what it is, is a tight fit for a 6-32. So what did the maker have in mind?
The washer that came off of your assembly must have just been a shaft spacer. For sure it wasn't going to help distribute the line loads into the shaft. If the button slots are offset from the center of the bellcrank plate (they always are) then there is a twisting moment on the crank. The joint needs to handle this twist without wearing out real fast and certainly without binding up. The thicker the actual bushing, the less of a problem this is. I'm kind of with the Motorman here. I wun'ta done it that way, notwithstanding that Bob H. sez these are pretty good unobtainiums.
Stuffing a crank into a thin wing happens on racing planes too, and can be a pain. I've seen plenty of planes that had the bellcrank arms bent down (or up, depending on which side you mount from) slightly to get the lines to line up vertically with the slot in the wing. The issue there is that this increases the offset, and therefore the twisting load against the bearing.
Hope you find a clean way to do this,
Dave