It´s a well written and I can see how much effort you put into it.
I also play assassin as my second main, mostly due to bg and the joy of playing objectives, while keeping 3-4 people at bay.
Same as you I also play shadow alone for the 0m restriction to enter stleath, facing the enemy´s back, so I can tell what the struggles, but also the perks are.
Starting off, you should be aware though that any kind and I mean any kind of class changes written in this forum are read, but will never be carried out.
Not simply because NCWest is not in charge of programming or doesn´t care, it´s because everything, aside from some very few occassions, have been chewed out and repeatedly requested in the Dev´s homeland, Korea. By the time we receive these patches 3-9 months pass.
Not saying that your effort is wasted, but if they went for sin, they would have to balance everything all together. From your post the majority of issues don´t even origin from sin. Warden, Archer(,FM, Destro, SM) - when you enter bg, the whole floor is lava, i mean aoe.
Melee classes since SSP patch were always at an disadvantage. While they had the highest risk, they had the least gain. Warden doesn´t count btw, with him being a mid range thingy. Still while I understand your claim, sin has many ways to bypass these restrictions, different from other melee classes.
Stealth basically disables 80% of all skills, any class has. Stealth can be basically considered immunity. All while one shotting classes that look the other way, yet still with the possibility to keep 2-4 people at bay. Choosing to play the objective.
Other classes are not that lucky. If they do not have that gear, all they´re left with is death. With the exception of BM light and spectral spec.
As sin you have to get away from the idea to kill and play for the team, delaying enemies by forcing them into combat, switching the brazzer while keeping watch, interrupting switch, providing party buff and saves, while trying to keep away from group fights, to avoid random aoe.
And still with the possibility to take people off guard with c stun, lbm (to get rid of the shield) and grenande.