The first sentence usually requires a comma. The second does not.

In their dressing gowns, Rose and Martin stand by the window (,) watching the scene unchanged from the night before: the storm continues.

Omitting the comma would mean the window is watching the scene. So, does it mean that the windows are watching? The comma clarifies that Rose and Martin and watching. No, one could imagine a window watching a scene, but try replacing window with a dog/cat would make life hella uncomfortable.

How do they reach the top of the cliffs where scores of youths are positioned (,) using the advantage of height for an aerial bombardment of rocks.

If you don’t want a comma in a sentence, it’s redundant because adding it is construed as changing

the meaning of the sentence.

Both sound completely wrong.

‘without success’ and ‘unsuccessfully’ both mean the same and can be interchangeably used in most scenarios.

I have tried to contact the customer without success” or “I have tried to contact the customer unsuccessfully” both imply your attempt (try) was without success/unsuccessful, whereas what you want to convey, was that you had tried to contact the customer, and were unsuccessful at contacting, not trying.

“I tried to contact the customer but was unsuccessful,” says Customer Support. I tried to contact clients but couldn’t. Sorry.