All of these are excellent points. I would add that poetic justice is served in another way, as well. Friar Laurence agrees to marry Romeo and Juliet because he thinks it might serve to end the families feud. Of course, it does, though not in the way he expected.
The conclusion of Romeo and Juliet, the denouement of the dramatic action, occurs in the funeral of the two lovers who are laid together in a vault to be close to one another in death as they were denied in life. A statue of Juliet is commissioned by Lord Montague to honor the Capulets, and the two patriarchal-led families are united in grief.
Justice enforced by the rule of law is often not consistent or even just. Poetic justice occurs when those who have caused the suffering of others are forced, by circumstance -not by law, to suffer themselves. Poetic justice occurs naturally as events unfold in one's life. There is no law, but Divine Law which enforces poetic justice.
The Montague and Capulet families' feuds kept the two young lovers apart and forced their attempts at uniting to occur through a plan that would fool their loved ones. This plan fails, of course, and the lovers take their own lives, rather than live without the other. The families are left to cope and suffer through their deaths. This is justice, because the families kept them apart. It is poetic justice because it occured without interference from the law.