There's has been a fair amount of discussion on other threads but airline rules do vary. Air Canada specifically forbids back to back and your description is correct. If you use all the coupons, it doesn't seem to bother them (ie do two trips) and doesn't seem to be back to back in their eyes. In fact, it was an AC agent that originally told me how to do this. I say if AC tickets it, then it must be OK.

There doesn't seem to be any problem with nested tickets and I do this regularly. After all, other trips do come up and you have to nest trips within trips sometimes.

Hidden city is another matter. I don't know if it is specifically forbidden but suspect it is.

Of course you can always make it legal and do a Mexican Hat Dance