My 18.5" is just over 21" total, with the comp on the end of it. Most muzzle devices for these things add about 2.5" of overall length. So, your 20" is gonna end up about 22.5" long for the barrel assembly.
That extra 1.5" of barrel isn't gonna get you much. Except more overall length to lug around.
Trigger. Think you bump-fired it? Are you absolutely positive that the buttstock was tight into your shoulder, and that you held the trigger to the rear after recoil?
I've got one on a 20" 5.56 precision gun - very nice stock. If you search out deals, you can find one for around $225. Sometimes they go on sale for right around $200. Gotta be quick to find them at that, though.
Here's the Primary Arms tutorial vid that they made, just for this scope. This reticle is pretty damn nice, and very easy to use. Can't say enough about this scope, and the price is unreal. I WILL be picking up more of these scopes.
There's really only one place to look for the ambi safety - Battle Arms Development. Pick your lever configuration. There might be others, but the quality and performance is unmatched.