Why is ARC in the game as an expensive mobile 1life class
Because an ARC ontop of their game can dodge every attack that 3+ players initiate at once. An ARC on top of their game is able to evade literally every attack in the game, you just need the skill to pull it off.
As for the rest of this, it's a few odd quirks that might need fixing (crouch mid air for stam regen of crouch) but there's nothing in here that is seriously game breaking, and complaining about ARC's playstyle just because you can't perform to that level is a L2P issue - not a balance one.
arc forward kick is a 50/50 strat unless the jedi you're playing against is smart and crouches in which case it's a 20/80 strat depending on if flinch stops them from sabering you (and seeing how bad roll into enemy as a strat is it probably wont)
rollback is better and keeps you away from their sphere of "lol ur fukin ded kid" and it just avoids the entire gamble of whether or not the enemy is bad
arc can absolutely shit on jedi if you abuse absolutely everything they have, but again, so can clones, and clones can do it longer (lv1 blob still >> their 1-2 emp nades they tend to have) and better (blob cant be reflected and even if it's expected, hard to react to) and easier (u literally blob them once and they die). once arc runs out of emp nades he runs out of his most consistent "i shit on u lmao" setup, and taking more than one emp nade means taking points of out dex, stam, or even something like armor (Please don't.) meanwhile clones get anywhere from 3 to 9 blobs (6 with default build) so between that and 2life they can seriously shred both gunners and jedi with relative ease (ofc, so can 3life solds with a single altfire frag each life, getting a free kill each spawn effectively)
If you run the grenade launcher, you lose even more dex/stam. Even if you do run the launcher the clone rifle just has better firepower most of the time with cr2 flinching the shit out of anyone who tries to touch you (God save them if you take cr3 blobs.)
Arc is basically a class where you can try really hard and end up being about as effective as one clone.
Too bad those things get two lives though.
The main issue here is your reflexes - regardless of how good a sith player is, they can do ZERO against a good ARC, an ARC with perfect reactions, prediction and understanding of how an enemy sith plays cannot be countered by a single sith - each sith player may require a unique playstyle from the ARC to counter but once that playstyle is set into motion, a sith has no ability that can hard-counter an ARC player, they have to rely on an ARC's mistakes.
I don't understand what your fascination with ARC's viability is though - sure the wallrun isn't always viable but that's why ARC has the highest maneuverability in the game (maneuverability, not flat-out speed) because if the wallrun doesn't work, you can use one of 15 other movement types to try and break through an area or - here's a thought - flank around and then use your crazy arc-fu to dodge shots while firing with perfect accuracy in return.
ARC requires you to think not only on where your enemy is but also where they are shooting and how to dodge said shots if you want to truly ARC-FU around and dodge hits left and right.
If you think EMP NADES are an 'i shit on u lmao' setup, you're bad at ARC plain and simple. Those things are overpriced garbage and exist only to troll Q-11/Goliath - troll, not kill.
I'd say practice playing ARC properly - a high mobility, high durability, high accuracy class for SUPPORT and dealing with saberists when your jedi goes down or drawing their focus from a group, because an ARC can evade literally every hit or attack a sith could attempt with ease, assuming their timing is on point.