The thing with the battle against the Mouth of Mordremoth is that it doesn’t just happen once. According to Laranthir’s dialogue box (whose contents change depending on if you’ve completed “Hearts and Minds” or not) the Pact fights against the Mouth at least twice, maybe even thrice (before, during and after final story mission). Here’s what he has to say about it:
Laranthir: (before completing final story mission) Our scouts believe the creature before us is feeding on ley-line energy for its master, but Mordremoth is an entity of mind and dream. That’s where you’ll have to go to defeat it.
Laranthir: (after completing final story mission) Though you killed Mordremoth within its realm of mind, the physical manifestation of its hunger still remains in our world, still feeding on this hub of ley-line energy.
This sounds just like the whole deal with certain npcs having different dialogue in living story depending on how far you’d gotten with the personal story. Like they call you commander if you’ve finished it, but not if you were halfway through.
I honestly took the second line as more of a “You killed it’s mind successfully, now we have to finish the job!” and not a “Hey, this thing keeps bringing it’s head back up!” What’s the whole point of “Killing him for good and stopping him from regrowing” if he keeps regrowing?
The extra lines of dialogue from sylvari perspective in “Hearts and Minds” show up in the mission’s respective wiki article. Basically the extra stuff regarding the battle is that the sylvari PC understands Mordremoth’s roar as “Armies cannot stop me”, which further proves the timing of one of the battles against the Mouth happening simultaneously with the infiltration mission, and it’s the one battle that has indeed taken place canonically against the Mouth.
Again, isn’t it brought up that Mordremoth can merely regrow his body repeatedly, and that killing his mind ends him forever? Still makes no sense to canonically have him return if that is the case.
There’s always the possibility, however, that either of Laranthir’s statements could only be taken semi-canonically and were simply written in to clarify the context of the battle for players who had or hadn’t done the story yet so as not to spoil the ending.
The first version (lines before completing story) indicates that the PC was present in the battle before sneaking into the dragon’s lair, which contradicts events shown in “Bitter Harvest” and “Hearts and Minds” where we’re helped behind enemy lines while the Pact’s battle is distracting the Mouth. I find it unlikely that the PC, after defeating Faolain, would just leave the Heart of Thorns tree and join the Pact’s battle against the Mouth, then returning back to the tree to finish the infiltration mission. Then again, we know from a dev comment on Facebook that a section of the Mouth battle taking place within the tree was cut, so maybe that missing piece was part of a previous version of the story which would’ve forced players to first defeat the Mouth at the tree before gaining access to “Hearts and Minds” but which was scrapped for timing reasons and because Anet didn’t want to force players to finish the meta successfully just to reach the final story mission.
It’s easily assumable that Bitter Harvest talks place the same time roughly as the lanes do, because the charr pilot mentions “The work done in the lanes has allowed him to fly up.” and gives transport back.
The second version (lines after completing story) indicates that the Mouth is functioning as the manifestation of the dragon’s hunger even after the dragon’s mind has been annihilated. Yet it’s contradicted by players hearing Mordremoth’s dialogue during the Dragon’s Stand meta (unless Mordremoth, while effectively braindead, would keep uttering its threats on autopilot). I put this discrepancy under “gameplay and story segregation” where the lines appear even after the death of the mind for the sake of epicness of the encounter.
Or, another possibility. The player finished off the mind before the dragon was fully defeated, and came out to help finish him off? Or the whole dialogue with the commander directly isn’t canon? Anet explicitly stated once certain events happen a grand total of once. Verdant Brink and Tangled Depths obviously happen once, so why can’t Dragons stand? Infact, I find the reasoning for repeated battles to be confusing. So what, the trees resurged and started popping out Mordrem and the three champions, despite us killing them?
I’d very much like to hear the writers’ statements on the order of events to clarify the matter and if we can count the pre- and post-final mission battles against the Mouth (as per Laranthir’s dialogue changes) as truly canonical or if they were done for the sake of player immersion rather than on the story’s terms.
IMO, his dialogue with the player (in sense as being commander) is probably non-canonical, given how the Commander never interacts with Laranthir after the first mission. After leaving the brink, the commander hardly does a thing as Commander of the Pact.