So if anybody is more involved into the mission than I am ... is my impression correct, or were there actual fallback mechanisms that these people could have decided upon?
They're all there monitoring the various systems because they're all responsible for various components of an incredibly complex piece of equipment.
Assuming the landing goes well, they all still need to be there to continually monitor the operation of the systems they're responsible for and provide feedback if things don't like how they expected - it's not just about landing there.
This is normal even when releasing a tiny piece of software; it's hardly surprising NASA do the same for a multi-billion dollar project.
But I don't think that means their assembly there was "just for show". That's the best place to see every detail of how well things matched plans: to see discrepancies, in context and with collaborating peers, as soon as possible.
And now that it's down, there could be info that requires adaptive action. Even though interpreting such new data, devising a plan, and sending instructions would take hours or days, it's still a "control" function, just a slow and deliberate one.
Additionally, watching a room full of engineers "do their thing" does a lot to get the average layman excited about the research and science and may get schoolkids excited about the possibilities of space travel, STEM, etc.
I don't think the control room was "nothing but show." They certainly could have all gone home, but given the time and energy invested in this mission over the last decade, It was importent to monitor the event as it happened even if it cannot be controlled.