In response to the first "yes", weren't there some older designs like that (double-barrel gun which could fire both barrels if the selector was mis-set)?
In response to the second, a combination of parts sufficient to build a machine gun is considered to be a machine gun. Does anyone have a double-trigger gun who does not have the parts necessary to join both triggers together?
My point #4 was that Charles Schumer says the people who called the MP5's were lying. Charles Schumer would never make any false accusations, would he?
Key words are "older" and "mis-set". New ones, IIRC, don't mis-set.
The "combination of parts" involves parts with an obvious intent to be combined for an illicit purpose. AFAIK, nobody makes a trigger-joining part just for that purpose. On a comparable note, IIRC, the ATF has recinded their infamous "shoelace ruling", as the shoelace is not a part made with intent for such use.
As for Schumer, "antis" usually don't know what they're talking about.