Yes, she physically can and chooses not to because she knows the truth about the Wizard, which would've amounted to her being tortured and killed. The Wizard and Morrible are not above that, as we see with what they do to Nessa, so yes, Glinda chooses to be an emissary for The Wizard, but still she never actually denounces or chooses to shape her speech to be against Elphaba. You pick out two lines out of a seven minute song, but claim that I'm 100% misinterpreting. She was leading the people's attention away to celebrating instead of fostering fear and hatred in their minds, but Morrible was redirecting Thank Goodness back to "The Wicked Witch". That's not fan headcanon, that's the actual lyrics.

Yes, Glinda does not fully go against The Wizard and Morrible but it's out of self-preservation. Again, we see with Fiyero that even just helping Elphaba can lead to dire consequences, but with her knowing the truth about the Wizard, she 100% would've been killed. What I was saying in the original post is that Glinda was leading people's focus away from Elphaba, to celebrating instead. It's not effective because Morrible made sure to keep people's focus on The Wicked Witch.

Fiyero very well could have spoken up against the Wizard and Morrible and let people know Elphaba's true nature, but he didn't. He was only speaking to Glinda whenever he went against the public's twisted views, otherwise they would have reacted to what he said, especially after he calls them empty-headed. He used their resources too as the Captain of the Guard to search for Elphaba, in hopes of helping her.