New member - have Q's re: oyster shells, what to do when hen dies

Hi everyone - I'm a new member who has visited the site many times and finally just joined today. I have been reading through lots of old posts and thought I would also raise my questions here in case folks had additional ideas or suggestions. I have raised four chicks (one each: rhode island red, easter egger, golden laced wyandotte, and speckled sussex), but sadly today one passed away. She was about 7 months old and had been acting listless for several days. I thought she was perhaps going broody because she had shown an interest already in lingering for awhile on the nest. But, today we found her deceased in the hen house. In hindsight I probably should have separated her from the flock and perhaps sought medical care but this is my first experience with a sick hen. The other chickens all seem fine but I'm wondering if I need to take any measures to try to protect them since we have no idea what led to the death of Winnie, our golden laced wyandotte. For example, do I need to clean out and disinfect the whole coop and run? Should I try changing their diet? I'm open to any suggestions....

I also have questions about whether/how I should supplement their feed. The chickens eat layer pellets made by Modesto Milling and then when we are home we let them free range in the yard where they eat a lot of clover, some rose bushes, etc. Do they need oyster shells or other calcium supplements? I thought the feed was supposed to be completely balanced and not need supplements, but I'm not sure how the free ranging affects the balance. If I do need to supplement, how much? Will they really self-regulate if I just provide it? Also, what about grit? There is plenty of it in the yard and also sand in their run (under straw) - is this sufficient or should I specifically add it to feed?

Thanks so much for any input on these issues to help out this newcomer!