Mike,
Sweet looking ride. Congrats.
In reference to your question about taking off the seat to mount the backrest; no you shouldn't have to do that. In fact if you do, you can't put the seat back on. The only way you wouldn't be able to put the back rest on, is that they forgot to install the mounting bracket on the rear fender under the seat. Otherwise, you should be able to install remove the backrest easily by sliding your hand down between the rider and passenger portion of the seat and pinch the two backrest legs together to slide into/pull out of the bracket. Open that area up and look in there with a flashlight and see if you have a kind of golden annodized metal bracket installed on your fender. They may just have forgotten to put that on.