The USN has no modern plans for militarized merchantmen and convoy escorts, unfortunately. During the Cold War they had decided on going the route of extensive dedicated convoy escorts, and that was the job of the OHP class. But those were all done away with for the LCS instead of getting a proper successor like they were supposed to; and now that the LCS has inescapably been proven to even those incompetent idiots to be a complete failure and a waste of time, they are scampering around trying to make a frigate replacement...
I cannot say for other NATO navies.
>How were armour plates attached and detached?
Special Bolts and Rivets. Very big rivets.
Obviously, it's more complicated than that and the practice differed not only between navies and eras, but between shipbuilders; so it's hard to give a straight and simple answer.
For example, one method was to rivet the armor plating to the supporting structure (I-beams, usually) through the (weldable) backing plate, and then weld the backing plates to the I-beams. Another method used bolts set into the armor plate when it was manufactured. Of course, you could combine these two methods, and I believe the USN did that with their ships. A fourth method was just to rivet the armor plate directly to the structure and essentially bolt the armor into the structure itself so it became part of it, this was the Japanese cruiser method.
>Did ships with external belts have a significant advantage when battle damage had to be repaired?
Yes, very significant. Replacing the armor belt of an externally armored ship was comparatively quite simple, as all you had to do was remove the belt and replace it. While not fun at all and rarely done, it was actually possible to perform this out in the middle of the ocean if you had the right auxiliary ships. With internally armored belts, you had to remove all of the covering material first in a non-destructive manner; you basically had to have a full on port-side drydock to perform this.