She might be simply nosy. Just because she ended the relationship doesn't mean she doesn't want to know what's going on in your world. It could be that she has a habit of looking at people's stories and yours is just one of them.
You could try reaching out to her in a friendly way and ask her why she's still watching your stories. But be prepared for any kind of answer, including a cold one. Maybe she'll be honest and tell you that she misses you a bit, or she might just say it's out of habit.
No, you don't need to be worried. It's probably just his way of staying updated on your life out of simple curiosity. He may not have any malicious intent.
It could mean he still has some level of curiosity or he's not fully over you. Maybe he misses the connection you had and watching your stories is a way to keep a small link to your life without actually getting back together.
There's also a chance that he wants to see if you're doing worse or better without him. It might be a bit of an ego thing. He could be thinking that if you're not doing well, it validates his importance in your life, or if you are doing great, he might be a bit jealous.
It depends on how you feel. If seeing his name pop up as a viewer makes you uncomfortable or prevents you from moving on, then yes, blocking him might be a good idea.
It could mean he still has some lingering interest or curiosity. Maybe he's not fully over you yet and wants to keep a bit of connection by seeing what's going on in your life through your stories.
She might be in a state of conflict. On one hand, she blocked you perhaps because of a fight or some negative feelings. But on the other hand, she's not fully ready to let go and still wants to keep an eye on you. It could also be that she doesn't want you to be able to directly interact with her, but she enjoys passively observing your updates.
Well, she might be having second thoughts. She ghosted you perhaps in a moment of confusion or because she thought it was the best thing to do at that time. But now, by watching your stories, she could be subconsciously missing you or just trying to keep tabs on you without fully committing to a conversation.