Well, it's hard to say. The story is full of surprises and twists, so it's difficult to predict Maki's fate exactly. You might want to keep up with the latest chapters to get the definite answer.
Yuji is still kicking in the JJK manga. The plot has thrown many challenges his way, but death hasn't claimed him yet. Who knows what the future holds though!
Megumi is not dead in the JJK manga. The manga continues to feature him and his adventures. The story hasn't reached a point where his character is killed off.
Miwa remains alive in the JJK manga. There are no events or plot developments that lead to her death. It's possible you might have misunderstood or there's a mix-up in your sources.