@JeffL thanks for trying to help -- I did have the tabs lined up correctly.
However what I was doing wrong was still embarrassingly basic: I wasn't pressing down hard enough. We were pressing down, and we thought pressing plenty hard, but still the metal just wasn't lining up with its track.
Thinking that in desperation we could freeze the metal part of the plug to make it fit, we disassembled the plug mechanism. While we were at it, we agreed there was no way the rubber seal was the issue ... but nonetheless tested putting in the metal ring without the rubber seal. Wow what an easy fit! If anything, disturbingly loose ... so this motivated us to just press down harder, which made it all work as expected.
So I feel pretty silly but really lean into the plug first then twist and it twists in with basically no resistance at all. The downward pressure in our case seemed extreme compared to what I remembered from last year, but I do remember needing to press down then, too.
Lesson learned.