The candidacy for a child under the age of 2 is a profound hearing loss. Over age two it is a severe to profound hearing loss in addition to limited benefit from hearing aids. That is generally measured by speech discrimination testing or by a child's spoken language not progressing and the child missing the developmentally appropriate spoken language milestones. If your child is hitting those milestones, he wouldn't be a candidate.
As for the risks, yes, there are risks. As for losing residual hearing, I have never heard of a story in which someone was implanted and they lost speech perception. There are lots of stories where they went from hearing aids to CIs and did not gain as much as they hoped, but I don't know of anyone who did not perform as well as they did with their hearing aids.
And as for the "pre-screening", there are centers that "rate" the child as a canidate. They label them an A, B, or C candidate. (And then an A, B, or C communicator). They do actually make an educated determination, based on various factors, as to what kind of benefit the child would get from the implant. Perhaps you could speak to some of the professionals from a center and they would be able to give you a clearer picture of the factors that make some kids successful auditory learners.