I"m not going to judge your vet because I wasn't there. I would have liked to perhaps have had some further diagnostics, perhaps imaging. Bloodwork is always nice. The question here is if the retained slug was the problem or not. Your snake very well could still have lungworms or other parasites; they are a common burden in wild caught snakes (and CB ones too). All in all, I would not expect retained ova to cause labored breathing.