Usurna possibly wanted to win the Tribunal’s trust, as well as fuel their huge fear of Gunmar (since they’re all basically too afraid of the Skullcrusher to even face him together, something Toby pointed out), by convincing them that destroying the bridge with Jim still inside is the ONLY way to keep Gunmar inside, even though he was basically right there with Enrique and maybe the trolls SHOULD’VE aided in his struggle to go through the portal, but Usurna probably forced them not too. This was probably just to motivate Blinky, Toby, and Claire enough for them to desperately rescue Jim by resurrecting AAARRRGGHH!!! by doing a few alternatives no trolls would ever do, knowing they’d do anything to bring back a friend (so she basically used their ability of improvising to bring back Gunmar without them being aware).
To the point, maybe Usurna WANTED the Trollhunters to regain the pieces of the Bridge, so when they did regain the pieces of the bridge from a sinking cargo crate, she waited until Draal (who was guarding the portal) had no choice but to enter the Darklands with the Gyre, so she quietly lead Gunmar and his now-blind lackey, Dictatious, out of the bridge, just before the Trollhunters returned alive. The Trollhunters coming back alive probably wasn’t a part of her plan, but she’s patient enough to keep on pretending until she convinced the Tribunal to blame everything on Jim for releasing Gunmar instead of being good leaders and actually talk about a way to defeat Gunmar, since Jim actually obtained the ONLY weapon capable of killing Gunmar.
Although the Tribunal is sort of well-meaning and want to do what's best for the world, they were absolute idiots for even agreeing on Usurna's tactics. Instead of actually talking about how to defeat Gunmar and protect Trollmarket, Usurna easily convinces them to just give the entire blame on Jim and you know what they did? THEY PUT THEIR OWN PROTECTOR ON TRIAL FOR JUST RELEASING GUNMAR, EVEN THOUGH IT WAS COMPLETELY UNINTENTIONAL!!! He even obtained the Eclipse Blade, which is the ONLY weapon capable of killing Gunmar, something no Trollhunter before him could've discovered alone. Usurna even proclaimed to the Tribunal that Jim has assassinated Vendel, even though he was in a cage the entire time, and do you know what happened? THEY ACTUALLY BELIEVED HER!!! And then they just had to realize their big mistake AFTER Usurna presents Gunmar to Trollmarket. You'd think that they could stand up and fight like true leaders, but no. Instead, Gatto cowardly runs from the mere sight of Gunmar and Wumpa made a deal with Usurna before she left Arcadia to protect her Quagawump compatriots (which is more understandable, because she had genuine care for her people, unlike Usurna). The unmaned member who spoke out to Usurna and Gunmar was a little more braver than the others, but then she didn't even have the guts to actually fight him, instead she cowered and allowed the Skullcrusher to turn her into a mindless Gumm-Gumm.
Basically, it was the TRIBUNAL'S own faults for even allowing the Gumm-Gumms to overrun their hidden world by just blaming the whole thing on their own protector (minus Vendel, of course, he always did everything in his power to protect Jim and even KNOWS that the boy is a more worthy protector than the rest of the Tribunal members thought). Heck, it was even a big mistake for them to even agree with all of Usurna's terms in the first place, and they never even stopped to realize that her actions were all just ruses that had absolutely no reason! They practically just disbanded their own entire council because of their own actions concerning Usurna's immoral perspectives (and because of their own skepticism about their first HUMAN Trollhunter, despite how many victories he's accomplished with his friends and close allies).
Most importantly, it was the Troll Tribunal's own skepicitism of Jim being their chosen protector that led to the ultimate downfall of Trollmarket (not counting Vendel, who has come to accept Jim as the series progressed).
"Either they were a lousy council, or just a bunch of dim, fraudulent trolls."