I'd say Gandalf and the Rohirrim is still a Deus Ex Machina. "Gandalf leaves to go get Eomer seven chapters earlier" isn't enough set up to not make it one, in my opinion. I think this Durkvelopment is one too, by the way. I'd have no problems with it being one, though, but I also think it was anti-climatic. Which is, honestly, a rather separate issue.
This is it. The heroes are delivered from peril by a vague extrapolation on the author's part. They did nothing to facilitate it aside from Durkon who... guessed? at the proper solution somehow. And then died, the end. I'm guessing there's more to it, and he ends up with Hel so this all serves another purpose, but that's just a guess and doesn't help things right now.
Re: Anti-climactic... it feels hasty even if that wasn't the intention. Maybe it's the sporadic timing of the comic itself, but the resolution feels like "cutting the gordian knot" where some things have to happen to keep the story going or wrap up plot threads so the writer/author just Gets On With It Already. That probably wasn't the intent, but the pacing brought on by the plot contrivance to get the story where it needed to be lends itself to it, anyway.
Compare to the last season of Warehouse 13, where the writers struggled to resolve a cliffhanger and give the series closure in 5 episodes, or the final episode of Agents of SHIELD where they had to wrap up a season-long arc and somehow tried to resolve character threads in the last 10 minutes (thankfully this might be fixed since they got renewed).