I guess Cha's speech confirms that there WILL be a way to get stronger for all the S-ranks, otherwise her scene would have made no sense at all. It will be pure bs when suddenly everyone can re-re-awaken, but I guess that's what it is.
You get "stronger" by learning Judo etc. You don't get stronger by getting better stats or getting new skills. That's why Go still trained when people asked him why he bothered when he's an S-rank, and why Tank dude said that the white ants were as strong as an S-rank, but hunters with coordination and thinking could kill them.

It seems like the stronger a monster (or a hunter) was in life, the more intelligent they will be as a shadow. Shadow-Iron doesn't speak, but he has got some personality left, even if it's not exactly the same personality he had in life.
Iron has a personality currently, yes. The shadows also apparently appear to have varying willingness to serve or have initiative on their own. Iron should be a pain in the ass for Jinwoo to control given Iron's dislike for Jinwoo, but that's not what we're shown.