To make the word order 'flow', as it were, I think a comma between 'of' and 'I' will disambiguate it.

It is correct and OK to use, though it is a more archaic expression by modern standards. I personally quite like it.

He shouts 'interlace your fingers' (that is, hold your hands so all fingers overlap the fingers of the other hand etc. etc.).

AFAIK, DGSE is the acronym for 'Directoire Generale de la Securite Exterieure' (insert loads of acute accents), the French equivalent of the US's CIA and UK's MI6 (broadly speaking). So a DGSE agent is basically a spy.

In terms of functionality, it's fine. Depends how 'nitpicky' you want to be, though. There shouldn't be spaces before commas (i.e. 'studied material, and also' etc.), and there's a case for expressing it as 'and we also', owing to how that's more natural to say, even though grammatically, both are fine. And 'Japanese hieroglyphics' may confuse people about the various Japanese scripts, unless you're referring to something else, so you may want to confirm what you're referring to as it may be better to say 'Japanese hieroglyphs' instead, though, I concede, this is only a point of pedantry.

Depends, before or during/after the ingredients are added and/or removed? Before, it'd just be referred to as water. During/after, probably most appropriate to refer to it as "soup". Though, since the Anglosphere doesn't really have an equivalent or analogue (the closest being 'fondue', but it is fundamentally a very different food), there's a case for calling it a number of different things with varying levels of justification, hence I tend to use "soup" (rather than "stock" and others).

Just a repetition, the stewardess stumbled when she tried to start the sentence, so just said it again, correctly/clearly.

"Sass" is more of an informal word, but it means an attitude, rather than being plainly rude. A more conventional word would be "feistiness" (usually used in its adjective form "feisty", though, as in "feisty attitude" which is essentially what "sass" means). Sassy people (that is, people who display sass or possess a sassy personality) aren't necessarily rude, but they 'talk back' (always have an answer) and use words that make the person they're talking to pause, and make people listening smirk. To explain, let's say a teacher has a "sassy" student. The student will give the correct answers in class, but try annoying the teacher at the same time (in a good-natured way, rather than being deliberately misbehaved). A "healthy" amount of sass means that the teacher will smile at the the student's efforts to annoy them. An "unhealthy" amount of sass means that the teacher will probably shout at the student for being silly in class. This is based on the assumption that the teacher is relatively good humoured.