1. Why couldn’t Newt just Apparate to the USA? Why did he go by boat?
Apparition becomes increasingly risky over long distances. As with most magic, much depends on the skill of the spell-caster: Apparition requires knowledge of the terrain to which one is moving, or the ability to visualise it clearly. Cross-continental Apparition would almost certainly result in severe injury or death.

Moreover, the beasts in Newt’s case had varying magical natures. Some could have Apparated with him, but others could not.

2. Why did Newt go in through No-Maj customs?
He was transporting magical creatures at a time when this was illegal. No-Majs were far easier to fool than the wizarding checkpoint would have been.

3. Why couldn’t Newt use ‘Accio’ to retrieve all his beasts?
‘Accio’ only works on inanimate objects. While people or creatures may be indirectly moved by ‘Accio-ing’ objects that they are wearing or holding, this carries all kinds of risks because of the likelihood of injury to the person or beast attached to an object travelling at close to the speed of light.

4. Why isn’t Veritaserum used in interrogations?
It is, but skilled wizards can avoid its effects by using antidotes and charms. A gifted Occlumens could also resist Veritaserum.

5. Why did ‘revelio’ undo the effects of Polyjuice Potion?
It didn’t. Grindelwald’s Transfiguration surpasses that of most wizards, so he used a spell, not a potion, to take on the appearance of Percival Graves.

6. Why didn’t Harry Potter develop an Obscurus?
An Obscurus is developed under very specific conditions: trauma associated with the use of magic, internalized hatred of one’s own magic and a conscious attempt to suppress it.

The Dursleys were too frightened of magic ever to acknowledge its existence to Harry. While Vernon and Petunia had a confused hope that if they were nasty enough to Harry his strange abilities might somehow evaporate, they never taught him to be ashamed or afraid of magic. Even when he was scolded for ‘making things happen’, he didn’t make any attempt to suppress his true nature, nor did he ever imagine that he had the power to do so.

And finally, an oldie but a perennial favourite…

7. Why wasn’t the Horcrux inside Harry destroyed when he was bitten by the Basilisk in Harry Potter and the Chamber of Secrets?

A Horcrux can only be destroyed if its container is damaged beyond repair. Harry was healed by Fawkes. Had he died, the Horcrux would indeed have been destroyed.