Your information is correct. They'll eat oyster shell as they need it and as long as they have a good diet with lots of fresh water available at all times you're good. If you have some extra eggs that are getting a little on the older side, you can also hard boil them and chop them up shells and all. Mine love that once a week. If you aren't feeding a mixed flock (roosters and chicks in with the adults) I'd also switch them to layer pellets or crumbles. More calcium in that than in grower. I've heard that veggies like Kale also contain some calcium. Mine go nuts over fresh Kale, although I'm just giving that as a treat once in awhile and not for any specific purpose like laying.
Rubber eggs can happen at any time. I had one pullet who actually laid two in one day, within just a few hours of each other. I physically watched her lay them both. Soft shelled eggs are actually less comfortable for them to pass. But they do happen. Even reliable layers can suddenly drop a shell-less or rubber egg out of the blue, then go back to normal after that. I still get one every once in a rare while. You aren't necessarily doing a darn thing wrong - just a hitch in the assembly line.