You must return all legendary permanent cards to the battlefield, even if the “legend rule” will put some right back into your graveyard. If any abilities triggered from the legendary permanents entering the battlefield, those abilities will be put onto the stack after applying the “legend rule.”

All of the permanents put onto the battlefield this way enter at the same time. If any have triggered abilities that trigger on something else entering the battlefield, they'll see each other.

You can't cast a legendary sorcery unless you control a legendary creature or a legendary planeswalker. Once you begin to cast a legendary sorcery, losing control of your legendary creatures and planeswalkers won't affect that spell.

Other than the casting restriction, the legendary supertype on a sorcery carries no additional rules. You may cast any number of legendary sorceries in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).