Grammar question: s-apostrophe vs. s-apostrophe-s

Hi folks! For anyone still around these parts, would you be willing to weigh in on a grammar question?

The question:

When doing the possessive form of a name that already ends in 's' (e.g. Remus), in British English, should one add just an apostrophe, or an apostrophe plus an additional letter s, or is either variant okay?

That is to say: "Remus' cloak" or "Remus's cloak"?

Generally speaking, both of these can be considered correct in English as a whole, but with varying preferences about it. I've looked at a few style guides and they literally all advocate something different. (Some go for the apostrophe-plus-s, some for just the apostrophe; some advocate both, depending on circumstance, with a lot of rules to govern which is used when, e.g. depending on whether the word is a proper noun or not, or on how the word is pronounced.)

All I want to know is:

In British English, is this a similarly free-for-all matter (preferences vary, but both can be considered technically correct) or is one definitely right and one wrong?

(Looking at the HP books, it looks like JKR does use the additional s with names that end in s, but not with nouns that end in s because they're plural – e.g. in the phrase "Tonks's parents' house" – extra s on Tonks (a singular name that happens to end in s) but not on parents (plural noun).

So. If I continue to follow my personal preference on this (which happens to be without the s, like "Remus' cloak" – I don't know why, it's just what my preference has landed on over the years) is that okay, maybe considered a variant, but technically not incorrect? Or is it essential that I change to writing "Remus's cloak" if I want to be writing actual British English?

Long post about a small matter, but I really do find these details fascinating! Thank you for listening.  :D
drunk, drunk Jian Yi, 19 Days, Jian Yi, Old Xian

Question on a slang phrase

Looking for the UK equivalent of "blown off".

"Blown off" in this context meaning ditched, avoided, rudely left hanging.

I was curious if there's a different phrase used in the UK, because all I've found on Google is that "blow off" in the UK actually refers to farting, which isn't at all what I'm looking to say, haha.

Any help is greatly appreciated! Thank you!

9/11 in British English

I'm beta reading a fic set in New York City in September 2002.  What would Harry call September 11th?  Would he call it 9/11?  The 11th of September?  Would he use the name September or the number Nine?

Athletics Professor or Games Professor, or Master?

Hey there all!  I have a quick question, if Hogwarts started a new class after the war like a PE or a gym class where the students were taught Muggle sports like football and tennis, etc., what would the professor be called?  Muggle Athletics Professor or Muggle Games Professor, and should it be Professor or Master?  I remember in Agatha Christie's Cat Among the Pigeone the position was called the Games Mistress.  That book was set in the 30s I believe, so the term could be outdated.  The only position I remember using Master or Mistress in canon was Potions.  Thanks in advance for your help!  Everyone here is always so helpful.  Please know how appreciated it is.
dark and scary

Birthday/Age celebrations?

I hope this makes sense. I checked the memories first, but couldn't find exactly what I was looking for. Also, my Google-fu wasn't up to snuff tonight. So here goes... in the US, we mark certain birthdays/ages milestones; 16, 18, 21, etc. In most states, you can acquire a driver's permit at 16 and some parents buy cars for their teenagers for this birthday. When I turned 21, I wouldn't let anyone buy me an alcoholic drink until I had bought my first one.

Are there specific British traditions for ages/birthdays? I found one site that mentioned presents are opened first thing in the morning when the child wakes. The cake is for later in the day and lavishly decorated. Small children are given "bumps". Loved that much more than our traditional birthday spankings. For older children, a house key was given to those coming of age signifying that they could come and go as they pleased, but that this had fallen out of fashion?

I know that JKR mentioned that at 17, magical maturity, the Trace was removed from underage witches and wizards wands, but what I am looking for are some other things Hermione might have shared with her parents. Muggle things that she might recall with fondness, something she might want to do with her children, or something that she and her parents would do early since her birthday fell on a day after she was already away at Hogwarts.

Thanks in advance!
Halo

Tail over teakettle, or ass (or arse) over teakettle

I saw this expression online when I was looking for British slang Harry might use in 1998, and I absolutely love it.  Does it fit with Harry and the time period, though?  For context, people crowded into a large stadium are pushing and shoving, craning forward for a better vew, and he thinks someone's going to pitch themselves over the railing.  I love the sound of it, but I'm not sure it fits the scene.  I think it sounds too light, where it's a heavy scene and he's very anxious and wants it to be over.  
drunk, drunk Jian Yi, 19 Days, Jian Yi, Old Xian

"Creep"

Just caught myself putting this in fic, and I wasn't sure: is creep a word that an older person from the UK might use? As in, "I'm such a creep", aka pervert.

If not, what are some similar words they might use in that context?
khalulu, kanji

"What's with him?"

Might "What's with him?" be used as a comment on odd behavior in British English (by any kind of British English speaker)? The context is just someone showing up and behaving somewhat oddly and leaving, and then someone else making a comment. Not angry or annoyed, just remarking that it was a strange way to act. If not, do you have any other suggestions? The one making the comment is not a major character so that's why I'm asking if there's anyone who might say it, rather than whether X type of person might.

Thanks, if there's anyone around to answer!