Definitely tape the magnets. I made that mistake with my ATA kit and it left scratches and marks I had to fill or paint over later. Big hassle that could be avoided. For the biceps and forarms, I did not use a cover strip on the inside but butted the seams and made a "weld bead" of CA then hit with accelerator. Then I used CA and E6000 to glue the cover strips on and it worked great for me. There are a million different ways to glue the pieces together so what works for some doesn't for others. For instance, some make their forearms clam shell with Velcro and others glue both seams and slide their wrist through. Same thing with strapping, but that is a whole different story.
Anyways to make a long story short, the only way to really know if a return edge fits correctly is to do multiple fits. A kit is never truly done as there will be something to fix or upgrade to make it either work, fit, move, or look better. Some people enjoy the look of a return edge at first as it makes the kit look more protective but can easily get in the way. Left some edge on my biceps and forearms but had to take them down as they were giving "armor bite" when I moved my arms.
Feel free to ask questions. I hope this helped.