With the poisoner perk from the pickpocket tree, do you get a bounty for killing people

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.

I've found that if you attack some one and they survive the first hit, they some how telepathically tell the guards, at least in towns.

A "stealth kill" is really any situation where you one-hit kill someone and no-one sees.

This is normally done while sneaking as you will get the damage multipliers (i think its X2 for bows and X3 for melee without perks) which make it a lot easier to do.

It's 3x for bows, 6x for one handed, 15x for daggers, and 2x for anything else with the relevant perks (Deadly Aim, Backstab, Assassin's Blade)