Goodness me, that's a long way to get through for it to fall through. What a difficult experience. If it was recently, I would definitely be taking some time to pause, regroup and reflect. I would also be asking your social worker what happened- if they weren't sure, they could have paused or extended the intros whilst any allegations were investigated and an accurate conclusion drawn. It seems so frustrating that the SW had so little sway over this relationship if they were all happy to proceed and had no concerns themselves.
What a shame for LO and your little family to be. Is there definitely no way that it could go ahead? The thing with FC, and I mean this in the nicest possible way to them, is that once the child has moved in with you, they genuinely have a very very different role to play in the child's life. It is less significant, because you are the ones doing the caring. Some FC can be angels, some can be very toxic, but regardless, you get to make choices about the future relationships you have with them once you are past that final hurdle.
I can imagine that if you removed yourselves it must have been a difficult and heartwrenching decision to make, and if you were the ones who made the decisions then it might take some time to process that. Introductions are hideously emotional, and the adrenaline that is seeing you through can take a while to dissapate and for you to come to terms with it in the long run. It might take some time- maybe an agreed break between you and OH might help him to not feel too rushed? It can be easy to feel you need to slam the brakes on if you feel out of control, whereas if he has time and space to come to terms with stuff, maybe talk it out with family and friends or a therapist, he might be able to view it differently. If it was recently, I should imagine it is all a bit raw and upsetting to consider something else right now.