Everyone, stay safe - Perfect and also commonly said as "Stay safe everyone" Everyone, remain safe - Also right but less natural Everyone, stay sheltered - This is correct but sounds like something the weatherman would say when saying to stay inside during a storm. You wouldn't typically use this in conversation. Everyone, remain sheltered - Again, technically right but awkward Nobody go out until the storm is over - Perfect and sounds natural - you might add "outside" depending on context All of your example sentences work without the commas as well.

These are all the first 4 statements are correct. Here in the U.S. especially the south "remain safe, stay safe" might be used more. Last statement was correct however use a Colon ( : ) not a Semi-colon ( ; ) because you started a short list. Very good work ~Robert N.Y. , U.S.A.