To me the biggest difference in Underhand Single Spey is the back cast sweep which can be done without dip after the initial lift which short line head makes possible. Centrifucal force has relatively big effect to the line sweep and adjusts the anchor distance and mono leader acts as a tail.
Purest Scandi style SS back cast is done without initial lift and sweep has rising plane. But sunk line head requires lift or one false cast which brings the line to surface which is often used to turn direction too so the the launch cast comes more a Switch Cast (Sparsed Spey).
Classic long line SS benefit of initial lift and dip on beginning of the sweep so that D-loop can be cast upwards in the end of sweep.
IMO many here pay a lot of attention to forward casting stroke. I think it would come naturally good immediately when Drifting is used. I saw Göran Andersson DH casting show about ten years ago and when someone did as him to make a long cast he did use both hands and long stroke. GA also dips more on newer videos he casts.
Stopping the bottom hand to belly is not necessary and I think it is not very common in Europe because it limits the casting distance but it can also cause skin irration and belly ache. Yes high stop narrows the line loop and it looks good but usually it does not improve fishing.
You are right and wrong on Perry Poke in Scandinavia. We do use the same cast but it was better known as a Byske Cast or Byske Loop and I recall it has Norwegian origin name too and here in Finland it is f***ed up Spey. I admit nowadays it is known as PP better but anyway it is very efficient cast when the leader is long and fly is not too big and goes about the same distance as SS.