How high can poison damage go

Lingering damage health does indeed stack. I tested this by giving multiple lingering poisons to an NPC via the poisoner perk. Her health went down noticeably faster when I gave her multiple doses. I do not know how the duration is affected as she died before the poison ran out. I will report back later after I have found a suitably healthy target.

After some testing, here is what I found out:

• All the potion effects are applied simulatenously. So if you have a poison that does 2 damage for 20 seconds and one that does 1 damage for 10 seconds, for the first 10 seconds, the target will take 3 damage per second, and for the next 10 seconds, he will take 2 damage per second. The end result will be 50 damage.
• The same poison will stack with each other, 2 poisons of 1 damage per second for 10 seconds will do 20 damage in total over 10 seconds.
• The reason why some people think they won't stack is if you give an entire stack of poison to the target at one time (any amount >= 6), only the first poison in the stack will trigger. The rest will be given to the NPC and sit in their inventory as a potion that they now can use. If you want all the poison effects to trigger, you have to give it one at at time (and risk the 10% chance of detection at each attempt.)

This also leads to the question that if I use the Concentrated Poison perk in the Alchemy skill tree, are the effects the same as if I used two of the same poison in a row?

This is correct.

Excellent build you have, that is my first character as well.

Summons work great (you only summon if someone notices you and you can't fire a shot off before they reach you). I personally felt the bound weapon conjuration perks were a waste. This is even more obvious after you raise enchanting.

I use a Daedric bow (enhanced with smithing), and dual enchants of fiery enchant(soul trap + small fire dmg) and I think paralyze if I recall correctly.

I'd much rather have those 2 perks back that I wasted on the bound weapons. Since the enhanced bound bow is nothing compared to a moderately smithed daedric bow. And then the fiery enchant at 1 second is a perfect enchant. Plus you can add another enchant once you get 100 enchanting(not add after the fact, but make another bow and add both).

I maxed Smithing and Enchanting, and was working on Alchemy. I personally think all my characters will max all 3 of those. Enchanting is nearly game breaking, and smithing makes you create ridiculous damage/armor. Alchemy is arguably as good as smith, but due to enchanting being so much more powerful than either, it doesn't matter as much.

Not to mention that I actually fully regret ALL conjuration perks. I still summon if the enemy(ies) are coming at me, but they are there to distract long enough for a shot, not damage. So having multiple (or more powerful) is a waste.