Yes.
This^^^^
If you don’t take care of your hands now they will not take care of you later.
No. Never. I consider myself fairly proficient and I've had a pretty successful career as a musician in studio and on stage, but at no point in my 40+ years as a practicing musician have I done 6-10 hour sessions in a single day. I guess that stuff about the point of diminishing returns happens a lot sooner for me and probably for most folks.
As to the practical matters, I live near the Rockies in Colorado USA, and we have a relatively arid climate year round, but really dry in the winter. If I don't take preventative measures my hands crack all the time, it has several triggers and it's impossible to predict when it happens. For decades I've been in a routine from around October to springtime of using Eucerin medicated hand lotion before lights out at night. Then during my morning routine I wash it off from at least the palms of my hands.
If I get cracks, I treat them with either a neosporin and band aid which promotes quicker healing, or I use one of the "second skin" products to seal it up until it heals. Second skin works best for the finger print areas, neosporin for top side and knuckles/joints. As I type, I have a dab of second skin on my left thumb and neosporin/band aid on the middle knuckle of my 2nd finger right hand. Try as I may, sometimes nature wins. But it is not interfering with my playing at all.
I would never play a stringed instrument with lotion on the palms of my hands. It will change the sound of your strings and lead to more quickly accumulating dirt and debris, and more difficulty removing it. It's harder to clean the neck and get it smooth and clean again. I keep my strings and instrument clean, wiping down thoroughly after every session and only playing with clean dry hands.