First of all, you need to discuss with your GM whether and how this works in the first place: RAW, you can only deliver a touch attack through a natural weapon if you are holding the charge, that is, you decline (or fail) to discharge the touch attack during its own action, and are left with the charge. Moreover, again, RAW, the rules for holding the charge all specify spells, and so would not apply to a supernatural touch attack like devastating touch. So RAW, you cannot do it at all, and even if we ignore that, having to waste a round holding the charge is simply not viable: don’t bother.
Many (myself included) consider this situation a mere accident of how they decided to organize the rules, not an intentional limitation. The rules come from D&D 3.5e, and in that game the Complete Arcane supplement updated them to cover more situations (though still not all of them—unarmed strikes can replace the original touch attacks without first holding the charge, but natural weapons go unmentioned and so RAW would not be eligible until the charge is held). When Pathfinder was based on the core rules of 3.5e, it copied the original rules but not the update in the (non-open content) Complete Arcane supplement. I am not aware of any direct commentary on the matter by Paizo.
So discuss with your GM whether or not you will be allowed to do this at all. In my opinion, non-spell touch attacks should be hold-able, and both unarmed strikes and natural attacks should be able to deliver touch attack effects regardless of whether or not you’ve held the charge first. But I am not your GM and I cannot make that call in your game.
If your GM does nix it, you can get around that with a conductive fleshwarped scorpion’s tail, since the conductive property explicitly allows this kind of thing.
Assuming the GM allows it or you use conductive,
- What damage will be dealt? Just the damage from the devastating touch (plus relevant class abilities like channel terror, endbringer's touch, etc.)?
Assuming you are Medium, the fleshwarped scorpion’s tail deals damage as a scorpion whip, i.e. \$1\mathrm{d}4+Str\$ slashing damage. To that base you would add:
\$1\mathrm{d}6 \times level\$ damage due to endbringer’s touch.
If you have used a terror that is channeled through the devastating touch, it would apply.
If you choose to use the poison effect of the fleshwarped scorpion’s tail, \$1\mathrm{d}3\$ Constitution damage per round for 6 rounds or until ended with a Fortitude save.
- If an ability/feature/feat/etc. states that it gives a "bonus to damage rolls," would those apply?
Yes, it would. You could even make the case that it would apply several times, for instance once to the damage roll for the fleshwarped scorpion’s whip itself, once to the damage roll for devastating touch, once for the damage roll of the terror you used, if it has one, and once again for the poison effect’s damage roll. My guess is most GMs would nix that, though, unless it’s a particularly small or particularly expensive bonus. Many bonuses will prevent themselves from working, either by limiting how many rolls they apply to or specifying something about the roll that rules out one or more of the above (for instance, only the base damage of the fleshwarped scorpion’s tail is “weapon damage” here).
- Would this allow me to apply the poison with this attack?
As already mentioned, yes.
- Would elemental damage that merely applies to any attack using the weapon itself apply here? Note: this is from the graveknight's "channel destruction"
If you had a flaming fleshwarped scorpion’s whip (I see nothing immediately that suggests you could not, though I am not very familiar with the fleshwarping rules and that may be blocked), that would certainly apply. The graveknight’s channel destruction ability would also apply.
- For the purpose of maneuvers or other actions/features that merely require "an attack" or "a successful attack," could this attack count?
Yes, this is an attack, and it’s successful as long as you hit and deal damage.
