Updated.
Does the environment work?
CR 1 for the spawn? They're tougher than a CR 1/2 stirge (and I've never seen a CR 3/4, 5/8, 9/10, or the like).![]()
Basically, the only thing they have going for them is their Empathic Suggestion, so it's all down to what CR you think that's worth.
...and DR 5/bludgeoning
...and immunity to crits and flanking and other benefits of being amorphous
However, since you pointed out their crummy mobility and less frequent blood loss, I'll concede they're even with the stirge (CR 1/2).
Well, we could drop the Damage Reduction. The original gibberspawn doesn't have it. Neither does the AD&D Gibbering Mouther or Greater Gibbering Mouther have resistance to non-bludgeoning weapons. For that matter, I wonder why WotC gave it DR/bludgeoning, it seems a poor fit to an amorphous, boneless monster - what aspect of its gelatinous mass with a single vulnerable organ would make it sensitive to blunt trauma?
Maybe they got it backwards and meant DR slashing & piercing? I wouldn't put it past them. Actually, didn't the gibbering mouther get a fix in the Draconomicon? I can't remember if they changed the DR...
Anyway, I'd be happy to drop the DR on the gibberspawn too and just give it CR 1/2.
As for the monarchy, I was thinking the 12 would be spread through the small village, not on every inhabitant. But either way works.
I'm guessing all the lips and eyeballs.![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.