UPDATE:
I just spent some time on the phone with one of the tech guys @ ARB USA, he was more than helpful! Below are some answers he provided as well as some I've dealt with in the past. Hopefully this is helpful.
Why the insert?
The spring eyes are handformed, they could have made them to fit the 35mm bushings just as the old springs did, however it was decided that by using an insert, the bushing would have a much more consistent surface to ride against, as well as be less likely to be damaged by the spring itself. According to the tech, it is pretty uncommon to have sleeve move, but the fix is easy. If you can pull the sleeve all the way out of the spring, do so, and slightly "crush" it in a vice to make it an elongated shape (very slightly). Then, press it back into the spring, this should hold it for good. If it moves a bit, but not enough to completely remove it, use a punch to "stake" it into the spring, the best location being right where the main leaf eye termintates as there is a small gap there.
Why the OME bushings with OME springs?
Some of the poly bushings on the market, are not made to the same tolerances of the OME springs, this isn't to say every manufacture, but there are some. This play can lead to premature wear of the bushing. Also, some are using a much harder poly material, while harder might sound better... its not always the case. The harder bushing is more likely to fail by loosing the "ear" than a similar bushing made of a softer material... make sense?
Why are my shackles not sitting perpendicular to the frame?
Well, the issue resides in your axle, not the leafs or the shackles (assuming your frame isn't tweaked at all). While the spring has a center pin that locates it into the spring perch on the axle... there is a slight amount of "play". When you install the axles on the vehicle in the air and then you sit it down, things might be mis-aligned slightly, and with the u-bolts tight its going to stay that way. The easy fix would be to put the frontend in the air (on jackstands) to that the axle is free-hanging. Loosen the u-bolts enough that you can "slide" things a bit... either by hand, or using a rathet strap between the leafs, "suck" them together. Make sense?
Eventually tech like this will be on my website, I have a huge back-log of similar info I need to get up there, in the meantime, ask 