Not too sure what the markings from 77-81 and 83-87 are asking for, my instructor thought some sort of pedal marking but we weren’t really sure what it was specifically asking for, and we don’t have a name for the symbol.
It's a pedal marking. Pedal goes down when the line goes down toward the diagonal line; pedal goes up when the line goes down away from the diagonal line. The line being diagonal has no special meaning aside from showing the duration of the sustain.
In this specific case, the goal is to sustain the low octaves while moving the left hand up to play the arpeggiated chord, but then release the pedal to accommodate the rest at the end of each measure.
Regarding the crescendo: the crescendo can be achieved while rolling through the chord.
He's probably thinking of the "cimbalom", basically a piano without a keyboard, Plucked like a guitar there is an AMAZING effect, where the player runs his finger to and fro along the whole range of strings, since the strings aren't "damped" they all sound at once, an AMAZING effect that makes your hair stand on end. Gypsy players would use this effect on an occasional off-beat. It is OBVIOUSLY what Brahms was thinking of in his G-minor Dance on the 2cd. quaver of the 4th. bar where he marks an accent which I've never heard anyone observe.