The best they can hope for

Even half retained, I don’t see Dadonov or Drouin getting positive value returns. Use those retention spots on Monahan/Edmundson/double retaining for another team.

You obviously don't follow a lot of hockey, as many as 7 teams have called the Habs this season about Edmundson. His value around the league and many ofthe others you have listed is higher than you think.

Retention isn't an option unless the return warrants using it. It's a waste of a spot and a 7th round pick isn't worth spending the amount of actual cash to acquire.
It's better to use the slot if possible of course, but you're just throwing money away at that point. A good GM isn't wasting their owner's money like that.
Dadonov and Drouin will in all likelihood simply walk to free agency and that's okay. We aren't losing value because it just isn't there.

I would be happy to be wrong, but was feeling pessimistic this morning

Its true, I think the biggest value they can provide right now is walking away in the summer and leaving us all that cap room.