First, it’s completely okay to post those questions in one thread.
In regards to your sentences, I think each of them is fine the way you have them written. However, in #1, A and B don’t have exactly the same meaning. Only sentence ‘b’ conveys your concern about leaving for the day. Sentence ‘a’ is correct, but not specific to the end of the day. He’s left, but it’s possible he’s still coming back later today, where sentence ‘b’ is clear that he will not be returning today.
In #2, it’s okay to say "I am afraid I can’t finish this work today’, but I think in spoken English you’re more likely to hear it in the contracted form of “I’m afraid I can’t finish this work today”.