It is possible to be rich and show lean if the mix is not being ignited. The sensor reads the oxygen not the fuel and if the mix does not burn it will read that unburned oxygen as a lean condition.

I think it would be worthwhile to throw a set of new plugs in, mabey one got fouled out then you started throwing fuel at it to cure the lean condition and wreaked all kinds of havok on it.

The single plane intake will give less signal to the carb and require a touch more fuel. I would try putting a piece of wire in the idle air bleed and see if that helps, if it does then you need to shrink the air bleeds. It sounds like your problems are with the idle circuit and since it is not makeing the proper air fuel mix then when you open the throttle there is not enough signal to get the other circuits pulling fuel properly. But again before I changed the carb any more I would check the ign system and throw in some new plugs.