Yes, you can wear them while sleeping, but there is a "danger" that the headphones will fall out of your ears while you are sleeping. But there is a special function that helps you find the headphones again. The function is available in the Galaxy Warable app.
That's also one of my biggest fear. Sorry to tell you that you have to by a new set. Every pair of Galaxy buds is synchronised with the other bud. Hope you will maybe find the lost one...🍀🍀🍀🍀🍀🍀🍀