If you are asking me, personally, whether or not I can justify ALL three of their actions at the tomb, then no I cannot. The ONLY person's behavior that I can justify is Paris's. He has a right to be there. His betrothed is dead and he is paying his respects. He sees Romeo there and confronts Romeo. Again that totally makes sense. Romeo has been banished and the penalty is death if he returns. Paris knows that Romeo murdered Tybalt, so he knows Romeo is a killer. He believes Juliet died out of grief for Tybalt, so Romeo has indirectly caused her death. It makes sense that Paris doesn't like Romeo, doesn't trust Romeo, and assumes Romeo is up to no good. So yes, it makes sense that Paris gets violent toward Romeo.
No I can't justify Romeo killing himself because he thinks Juliet is dead. I know that I'm a bit cynical when it comes to their love, but I can't help but see both of them as melodramatic teenagers. Juliet is 13 years old. Romeo's age isn't specified, but teenager is assumed. Three days earlier Romeo was completely destroyed and depressed that his dear Rosaline didn't love him back. Then within 3 days he's madly in love with another girl, marries her, and is willing to die if he can't be with her? I'm confident that Romeo would get over Juliet, but alas, we'll never know.
I feel the same way about Juliet. I have no doubt that she probably loved Romeo deeply, as much as you can love someone you met two days ago, that is. But no, I don't think she is justified in killing herself if she can't be with Romeo. That's an incredibly selfish act on her part and caused extreme heartache and sorrow for anybody that knew her. I suppose the argument could be made that because of their deaths, the family feud ended. But it's also possible that the feud would have ended because of their marriage . . . like Friar Lawrence hoped.