Return edges can be a sore subject. Are they required? No. I have very few left on my armor for comfort and sizing purposes. However, some people really like them because they give the illusion that the armor is thicker than it really is.
As for the glue issue, the problem with ABS is it melts the plastic together so even if you have it on just one side and goof off would start to remove it, what you removed would be uneven. I suggest sanding. I also suggest getting some Rockler sandpaper (rockler.com). They sell sand paper pads of a very high grade (through 12,000 or something crazy). With that stuff you can actually sand the plastic back to its original shine. Here is an example, you can see I got ABS on the bucket. This also happened on the front of my bucket. just above where the mask meets the dome. I was able to sand it out completely: