Both are absolutely fine, and sound natural - I couldn't even tell you which I'd be more likely to use if I were to. If there is a grammatic rule governing it, it's unlikely that the reader would be able to appreciate it.

Very little - mainly in the spelling, so far as I know.

'Mutter' is the normal one for 'under his breath'. 'Mutter' is usually complaining to yourself - if you've been told to do something but don't want to, but have to. 'Murmur' is quiet, similar to whisper, but not as conspicuous - for example if you wanted to make an ironic comment to a comapanion when...

None or very little when talking about their use for capturing humans/animals. I think that 'snare' might also be a surgeon's instrument, but I'm not certain. Trap may also be used more metaphorically: He fell into poverty's trap .

It's possible that it was referring to a '99 Flake' - a Cadbury's chocolate bar, which is usually assossiated with ice-cream - a small '99 Flake' would be pushed into the ice-cream - these are most often bought from ice-cream vans. Sometimes this is shortened to just '99'.

Can you give the context? Edit: my gut feeling is that the former is correct, but I'm interested in seeing the context

I would say that present tense is quite acceptable too, describing a book as it is - as it still exists. However you must remain consistent in your tense use. Sometimes you would want to use perfect tense in mainly present tense essays, and sometimes pluperfect in perfect tense essays, but these are...

I'm a native English speaker, but this doesn't mean I'm infallible: I would say that in 4 and 5 either could be correct, certainly either would seem natural in spoken English. Number 6 doesn't seem grammatically correct to begin with, you'd need to have "since he was young", and I would go for 'gotten...

Amandine is correct, but it isn't limited to this. One could say "It has been raining, but now it's sunny" - meaning recently it's rained, but it stopped and is now sunny. A time period isn't necessary, and it doesn't necessarily mean that it's still raining.

Thank you, that clears things up - so in the first given examples it should really be 'besides', right?