gmay3, adding a GT2 adapter (like you linked to) shouldn't affect the accuracy, so long as you re-tension the drive belt.

But I'm wondering, isn't there a real, machined GT2 bushing/pulley/bearing that exists? If it's one with guard walls, it may be easier to alleviate the problem at hand.

As to the core design problem with the Y-idler belt rubbing, I think the mounting brackets on the bottom of the Y-bed are what's out of alignment (shifted to the LEFT of the i3v), as I've had to move the GT2 driver further to the end of the shaft on the stepper motor to compensate. This makes the problem on the Y-idler even more pronounced, as now both the bed and stepper are 2-3 mm further to the left.