I can tell you for a fact they are NOT adapting the remaining chapters in the remaining episodes. I'm sure of it.
The chapters in the manga only grow and get more substantial. The pace started off at 3 chapters an episode, and is now at 2 chapters an episode.
I fully expect to see a single chapter episode, eventually.
And Trigger has already broken their no more stuff of same IP rule with Dynazenon, Gridman Universe and others.
The reason they don't continue stories they have finished, is artistic integrity. The same reason leaves me completely doubtless, they will not be rushing Dungeon Meshi or doing it dirty in any way.
Worst case it's a financial flop, and they don't finish it, but I fully expect they plan to.