Of course, he can. The rejection was a momentary decision. It was circumstantial. If he gets to know you better, you guys can have the chance of being in a relationship. But accepting him after he rejected you is totally up to you. After all, this isn’t the Regency era.
Your ex is trying to study your expression and make you uncomfortable. They aren't happy that you have moved on in your life so easily. Perhaps they are not at peace. Try to ignore them because they belong to the past, and you have a wonderful present to work on.
She always had a soft corner for him, and it shouldn't surprise you. After all, you can't understate that something about him moved her to cheat on you—priorities what matters the most, the peace of your mind. If you feel wronged, learn to forgive and move on.
They followed you just because they wanted to know how you are holding up. If you want to know more about your ex’s life, you can follow them back. If they have a public profile, you can keep track of the person without their knowledge. Just don’t watch their stories.
Your ex is taking their time off to overcome the breakup before making a reappearance on social media. At this stage, they are not interested to engage with their friend circle on social media. You must not let it disturb your inner peace. Focus on your growth. Make yourself better.
Your ex hasn't deleted your number because they don't want to break all the connections with you. They hope to understand and continue on friendly terms without any unnecessary bitterness in between. It's the mature thing to do; deleting numbers won't help unlove them.
If your ex hasn't changed her relationship status, she is still taking time to absorb the reality. It all happened too fast, and she might be stuck in the past. Allow her some time, and once she has accepted the reality that you won't be seeing each other anymore, she will change her status too.