I don't see a problem at all using a rivet under the sniper plate.
I really don't see a benefit either, tho.
I also used scrap strips on the back of the butt joint(TK5270's AP). Tho I did use finishing strips over the top as in ANH. Not sure if there are nice shiney for sale signs out there for this, but maybe there are..?
As for glueing your sniper plate...be sure to use E6000 for most of the securing of the plate. You could tack the sniper plate down with CA glue to hold in place, then use E6000 generously to provide the best holding power. E6000 has some give to it allowing the plate to be bumped or even "leaned on" lightly for a short period of time without having the plate pop or snap off.
I would email some of the armor makers asking for a quote on the finishing strips. If you're gonna do it, do it right. The look will improve greatly if you use the proper strips. IMHO
Enjoy the build.....and......Where's all your pictures????