Sorry for the delay on the promised details. Some more observations...
First let me be clear that I did not write the WPA rule with the "ball-return system" language. Mr Jewett could tell you who can take credit for that language if it was not he.
My credit is the BCAPL/CSI rule which, unfortunately, does not contain the ball-return system as part of the definition. However, the intent of the BCAPL/CSI rule when written, although missing the return system language, would definitely consider the ball as pocketed. If you insist on an analysis of what part of the particular CSI/BCAPL language covers the situation, you kind of have to back into the ruling by looking at two things.
First the similarity of the ball leaving the ball-return system to the established and specific CSI/BCAPL Approved Ruling for the definition of "pocketed Ball", Situation 4. Fortunately, the phrase "ball-return system" does appear here:
Situation: A ball drops into a pocket without rebounding from the pocket, and then falls to the floor because of a flaw in the pocket or ball-return system.
Ruling: The ball is pocketed.
The similarity to the OP situation is more than close enough. This retired referee, at least, considers that any ball return system that is designed such that the OP situation is even possible is flawed. And the end result is that the system did not perform properly by retaining the ball. To consider the ball as rebounding from a pocket to have more weight than the similarity of the OP situation to the Approved Ruling is, if I may be so bold, a ridiculous stretch.
Second, the clause from the CSI/BCAPL definition of "Pockete Ball" that states" "A ball that rebounds from a pocket back onto the table bed is not a pocketed ball." As the writer of the sentence, I can tell you definitively that the phrase "rebounds from a pocket" does not contemplate a pocket other than the one that the ball initially entered.
An Applied Ruling that specifically covered the OP situation was on the shelf when I retired, but will likely never see publication now.
For those wondering what to do with the 2-ball, it goes down. Had the 2-ball contacted or disturbed other balls after returning to the table, I would recommend application of CSI/BCAPL Rule 1-48, Non-Player Interference, or WPA Rules 1.9, Outside Interference, and WPA Rule 1.8 and Regulation 11. Restore if possible, re-rack if not possible.
Again, since I a retired my estimations and analyses are no longer official. I am confident in them, but the CSI office would have to answer for an official ruling concerning CSI play.
Buddy