That's a very uniqute interpretation of alignment in the fallaway - I've had it taught that way, but also know more of comparable reputation who would strongly disagree and argue that the fallaway must track straight, with internal twist between body orientation and direction of progression.

DRS in Foxtrot ?
is this very advanced or an American thing ?

I wouldn't say it's "very" advanced, it's just not in the ISTD syllabus. But if you can handle a double reverse spin with waltz timing, you should also be able to do it with foxtrot timing. It's the same step, after all.

LLuv2dance. Iìi come into the CBM debate with a question. Does one step and then move the body into CBM. Or does one do the CBM and then step. Or are the two done simultaneously. Just on the first step of a Feather Step will do. or the first step of a Reverse.
For my money the step goes before the body every time otherwise at full stride we will never get our body weight to a split weight position. by that I mean right smack in the middle whilst still moving.

Split weight position never occurs when actually performing most swing dance figures - it's an artificial static position usefull only for explaining other aspects of the figure, such as step placement or rise and fall.

In the swing dances, you must never let a leg reach in front of the body - to do so would be to put it in your partner's space. Thus CBM occurs as you send the body into the step, unlike in tango where you twist the body, reach the leg, and then move onto it.

It's a common fallacy to believe that you will get a bigger step by sending your leg ahead of your body on step one; in fact, all this does it put you off balance, making you place step two - the one that should be big as the swing uncoils - too soon, which is another way of saying too small. Step one is a preparatory action only - it's step two that goes somewhere.