Carb experts questions

Well, I for one would fix it.And it may not be easy.
But if you don't fix it, it will never run right as you cannot get the T-port sync right. Some 80% or more of the life of the engine, in a streeter, is spent on the low speed circuit, and a great deal of that time, the idle ports are flowing. So if you can screw a mixture screw all the way in without it affecting the idle speed or quality, I would be sending it back if new, or if used, I would be tearing it apart.
It could be as simple as dirt or debris in the air bleed or in the delivery circuit, or a bad gasket. Or it could be a more in-depth problem in the idle well, where,if it's loose it can get into the transfer circuit, and that could make your life miserable.