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).  
		 
		
	 
I don't see how they're tougher foes, at least in a physical sense.
Stirges can fly at 40 ft. and drain blood at 1d4 
per round as a standard action. The Spawn moves at a 10 ft. speed and can only drain 1d4 Con 
once per day as a full round action. Further, the Stirge has better AC (16 vs 13), much better attack (+7 touch vs +2 melee) and arguably superior saves.
The Gibberspawn doesn't have any Hide skill, so it's trivial to spot and kill one as it 
ssllloooowwwwlllllyyyyyy crawls towards its opponents.
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.
Physically, they're a CR 1/3 at best, but being able to use 
suggestion on their host at will makes them a worthwhile threat. The SRD 
Nixie is CR 1 and can 
charm person (DC 15) three opponents simultaneously, that's a lot more useful in a fight than the gibberspawn's ability.
So I'm backing CR 1/2 for them.
EDIT: The Psionic Handbook's CR 1 
Puppeteer seems more similar to a Gibberspawn than a Nixie, but it can 
dominate its host, not merely 
suggest to it. Furthermore, it has 3/day 
psionic charm and a great Armour Class (AC 20).