This is, quite frankly, not true, and definitely not what BW intends. Some of our raid's top DPS are non-tank shadows, and Vanguard and Guardian DPS is respectable as well, though perhaps not entirely up to par with sentinels (which are a bit overpowered atm), which might be why you don't see then as often. If you want to dps as a shadow, vanguard, or guardian, you certainly can do so, and with the right gear and group experience can clear any of the content in this game.

All of the tanking-capable classes have 100% viable DPS specs, just as all healing-capable classes do. Thus, all classes, when specced correctly, are capable of filling the DPS role.

We've found that it helps to let Toth jump and then return to his original tank before making the switch. This is probably less than a 5 second wait, but we've found it makes the taunting issues easier / nonexistent, and the Zorn tank can probably survive the 2-3 hits he takes before charging over to Toth.

You just make sure the melee stay away from Zorn and dps Toth down, only getting out of his AOEs when he does his Berserk. What's the problem? A mostly rakata sorc healer can heal the tank and 2 melee dps through this fight on HM (ours does).

The first boss is hard, but it is doable if everyone pays attention to the tank swaps and stays out of the avoidable damage. My guild did it on 16 man story mode with most people in columi / rakata and at least one person in columi and worse (he had a lvl 27 green item...). We made sure the ranged dps stayed on Zorn, and stood far enough away from him to not get hit by his dots and stuff, while the melee stayed on Toth and only backed off when he did his berserk jump smash thing, as it hits a lot harder than the other jump smashes. When the red circles show up, you get out of them. The tanks switch every time Toth jumps to Zorn and debuffs the Zorn tank. On story mode, Zorn and Toth require your tanks to be coordinated, your dps to know how to position, and everyone to know how to avoid the giant red circles. DPS is not even much of an issue. On hard mode, it's pretty much the same, but DPS is much more of an issue, and bad positioning by dps is punished by instant death. These fights don't need to be nerfed, people just need to get better at the game / get better gear.

Is this KP reg, or HM? If it is HM, then I'd say Bonethrasher HM is harder than any HMs other than Soa HM. You probably need everyone to have mostly columi / rakata. I assume you are hitting the enrage timer, rather than just wiping to swipes? If so, then you are probably losing too much DPS running around avoiding swipes. You seem to be going the fight correctly, just try to have everyone focus on doing as much damage as possible while staying out of the swipes, and killing the adds he tries to eat (the little ones). When the big adds spawn, CC them ASAP, and continue the DPS. If they get un-CCed, have the tank taunt them, and then reCC them and keep DPSing. The whole fight comes down to whether or not you have the dps to kill the boss before the enrage, the positioning to stay out of the swipes, and the healing to give you some leeway on the swipes.

I'd say TV, MP, FE, The Ess, D7, KuS, Ilum, though some of those could probably be switched around. TV is super easy, and you can skip both mini-bosses and most of the trash if you pay attention and hug the walls. MP and FE are almost stupidly easy once you do the first boss of each. This goes for the Ess as well, but not so much now that the first boss got nerfed. D7 mentor can be annoying, and the first boss glitches a lot. KuS is probably actually the hardest one, as far as mobs and mechanics go. Pretty easy to wipe on a lot of the trash in there. Ilum i say is the hardest only because the enrage timer on that boss with the spwaning adds 9the 2nd one, i think) is really short, and if your group has too much melee dps it can be a real pain. Everything else in there is not that hard. This is of course for republic.

As a Vanguard, I never actually switch stances, so...i'm not really sure, lol.

I would suggest everyone lvl up synth/armormech and make the bracers/belt until they crit for augment slots, then switch to artifice and do the same with relics. Takes some time and money, but they are currently the best in slot for their items.

You are fine. Find a decent healer and start with False Emp (it's easy and has one of the best drops).

I'm in mostly columi/rakata PVE gear and I have 21.4ish k when i pvp with my own trooper buff and dps relics. If i had battlemasters stuff i'd probably be about the same? I'm not sure what the max would be with full rakata/battlemasters and stims, but im sure its 23-24k, and is probably quite scary, lol.

If you were facing 3 dps and 1 healer, who would you target first? Probably the healer, and then focus down the dps. What if one of those DPS was suddenly a tank, with guard on that healer, effectively giving the healer double HP? It takes a lot of damage to burn through a guarded healer (usually 33k or so health, assuming they have 16-17k health normally), in which time they can heal themselves, and the dps can kill you. If they target the dps, the tank can just move the guard, causing the same problem, and/or the healer can help negate a lot of that damage. Taunts also cause a lot less damage to be taken as well. So, you could target the tank, which IMO may actually be the "best" choice, as without a guard on them tanks technically have less health than the other members of the group with a guard, and without taunts you do the most damage to them. The problem here is that even without guard a good tank will have 18k-22k health, defensive cool-downs, and generally take less damage with armor and defensive talents that reduce damage taken. So, attacking the tank still takes a long time, and the whole time you still have the dps chewing at you, and the healer healing the tank. Tanks also do relatively decent damage, so that with their defensive abilities and larger health pools can usually go 1v1 with most other people if needed. A healer and a tank can hold a door or turret for a painful amount of time, allowing enough time for help to arrive, or other things to be accomplished elsewhere on the battlefield.