I didn't mind the ending, especially after I realised that this wasn't really a story about some guy getting kicked out of the hero's party. Turns out it was actually about the birth of a demon lord. Overall IMO it ended better than it started.
As for Allen's fate, I wouldn't be surprised if his last gasp basically burned up everything that was left of him, leaving nothing behind for the dungeon to absorb. I kind of get why some people are saying that dragon-girl is allen-reborn, but I don't believe it myself. Rick's synthesis needs to merge two monsters (maybe people work here too), but there was nothing left of Allen to merge. And even IF it's new-Allen, it'd only be half, since monster synthesis needs more than one 'ingredient'.
I do like that the healer Mary go out of it all unscathed, including the political bullshit side of things. I do wish that creepy alchemist guy and the king got some of comeuppance though.