Hmmmm, something not right with that one Zeb, The VIS
motors you returned had both been tampered with previous, one was rebuilt with the cam in 180 deg out and had never worked since, the other was what looked like just some old bits that had been put in to fill the spaces up, total scrap. So, it may be the manifold was also messed about with.
People ask me often how long a Vis will last? It's allways difficult to say how long as you are never sure what enviroment it has to work in. If an engine does not "breathe" well it may have more oil than is healthy for the vis and kill the motor really quite quickly. But, that may assist the butterflies. Other end is very little oil ingress giving a VIS
a long and healthy life but as you say that may not be good for the linkages. Shame we have no data on the subject