Hi mate,
Hopefully this helps.


Q1.
I don't think you need to over think this.
The technique they want to employ is swinging the flies at the end of the drift. It makes little difference on the angle you cast in the beginning.
To be exact though, the author suggests an angle greater than 45 degrees as this would be slightly down, so perhaps 50+ degrees. If you are still confused I'll put up a diagram.
Make your cast and let the current take the flies and let them swing at the end of the drift to achieve what it is you are trying to copy.

Q2.
That makes no sense at all.
Sea run trout are the same as brown trout except they are in estuarine water.

What I am perhaps reading into this is that the author might suggest they have softer mouths than salmon as a lot of anglers target salmon and catch the sea run browns as a by-catch on surf casting gear and rip the mouths off the fish with aggressive techniques.
The sea run browns definitely have softer mouths than salmon. In fly fishing, the mouths on browns hold up just fine to a hook unless you winch them in.

Q3.
Your understanding is correct.
The fish swallows the fly and the is hooked deep in the mouth or throat and the nylon connected to the fly is rubbed by the teeth or the rough mouth parts and it damages the line.
It usually doesn't take long to rub through if you are fishing very light tippets.

In my limited experience works best if rod is swung almost parallel down river the same as is often done after the normal drift whilst nymphing is almost exhausted & in preparation of the next upriver cast.