First order of business is disabling the interlock
As the rules are written, there's no mention that the advantage is explicitly limited to single ball play in MB:
"If a ball becomes stuck during a multiball mode, the player should attempt to trap the other ball(s) in play and request assistance. A stuck ball during multiball often represents a significant beneficial malfunction, and intentionally taking advantage may result in a penalty."
Sometimes a stuck ball in mulitball doesn't offer a significant advantage, sometimes it does. The rule as it's written doesn't care. It's all about getting that ball unstuck.
"If the game is in multiball play and one or more balls are lost as a result of freeing stuck balls, possibly ending multiball but not ending the ball in play, this will be considered no worse than a minor malfunction."
So BY THE BOOK, that player is fucked. They could have 8 balls trapped on their flippers, with a ball stuck, and without the interlock disabled, the freeing of that stuck ball will result in the end of their multiball . . . sorry.
With that said, major tournaments typically have those interlocks disabled so this kind of situation is a non-issue. For SMALLER events where this situation is far more likely to occur, I would have no problem with a TD exercising their paragraph 10 right:
"Final authority for any ruling, including rulings that contradict or vacate anything written in this document rests with the TD."
If this happened at our selfie league for example, I would totally Paragraph 10 this shit and have them shoot the goal and open the coin door as soon as they do. The other example of allowing the player to play on with 2 balls is 'fine', and if they drain down to one ball just have them junk it. I would probably add the caveat to the player that if they DOUBLE DRAIN those two balls in play, I would dump the stuck ball as well ending their ball. They are responsible for getting control of the ball in play so that risk is still on them (which is the advantage to playing with that stuck ball, no risk of actually losing your full ball in play).