Inconsistencies in Harry Potter

The family and I all love the Harry Potter stories. But that doesn't stop me from thinking about the loose ends I spot. Not really a serious criticism; if anything, there seem like relatively few loose ends for such a sprawling story. But it is still fun to note them. Even more fun if someone points out why I am actually wrong. They are lsited below in the order they happen to have occurred to me.

1. In Book 4, the accused receive a form of jury trial, and it seems clear there is nothing different between wizard juries and muggle juries--the verdict is regarded as subjective. However, later in the same book, "Veritaserum" (truth serum) is used to get Barty Crouch, Jr. to spill the beans about Voldemort's conspiracy to be restored to his body. So why didn't they just give Veritaserum to the suspects on trial?
2. Why don't wizards use guns? Obviously, it would take a lot of the "romance" out if they did. Tolkien gets around this because his stories are set in a pre-industrial age. Frank Herbert, in the Dune stories, had a very clever way to justify modern people fighting without firearms--people had "force-shields" that would block any fast-moving object, but which could be penetrated by slower-moving objects (e.g., a well-calibrated sword-thrust). Rowling provides no theoretical explanation for this in her books.
3. The primary plot-line of the fourth book, involves Harry's unexpected participation--achieved through trickery--in the lengthy trials which culminate in winning the Goblet of Fire. The object of the final trial is to lay hands on the goblet itself. Unbenknownst to anyone else, one of Voldemort's helpers (Crouch, pretending he is Moody), has bewitched the Goblet, turning it into a "portkey"--a sort of magical transporter, which, when touched, will transport the toucher to a pre-programmed destination. Wouldn't it have been simpler and surer for Moody to have created some different portkey, and easily found a pretext for Harry to have touched it?
4. Introducing the element of time travel in Book 3 worked neatly in the context of that book's plot, but I think it opens up a can of worms. Why not use time travel any time something bad happens that you would like to undo? For instance, in Book 4 when Harry and Krum stumbled on Barty Crouch, and Harry went to get Dumbledore, only to find Crouch had disappeared when they returned, why not employ time travel to prevent him from disappearing?