I did know that the fake silencer on the GSG was against NFA regulations (for what reason nobody knows) but that is only on the GSG-5 SD model. Mine is the plain 'ol carbine GSG-5. If I could guess as to why the ATF would call it illegal, I would think that since it can be added onto a shorter barrel, replacing the flash suppressor, the hollowed-out portion could be utilized as a make-shift silencer. But that's just a guess.
As for short rifles, from what I gather from your post, if the overall length is greater than 26 inches AND the barrel is at least 16 inches, it is a rifle. Any shorter length on either one constitutes a short rifle.
I'm pretty sure that I'm going to get the SBR tax stamp for the rifle regardless. I guess that I am reading into the Gun Control Act a little too closely as well. I keep going back to it and it clearly states that regardless of how long the barrel is, as long as the overall length is greater than 26 inches, it is not a short barrel rifle. I would be absolutely fine with the whole definition if they had used the word "OR" instead of using the word "AND".