Caltrops for the slowing effect, no. Caltrops Jagged Spikes for the DAMAGE effect? Yes. You can stack a maximum of five.
@ OP, I think the confusion of the Life Steal effect with Shadow Power Gloom with Jagged Spikes is that I realized that even though I have "Healing Numbers" shown (Green numbers float if I get healed), it does NOT show the healing done from Jagged Spikes with Shadow Power activated. I suspect it's because the numbers would flood the screen and just leave green streaks clogging up the screen.... I don't really know. I do know that my health globe fills regardless of the fact that I'm doing nothing else but kiting through Jagged Spikes.
A matter of language:
Proc = an effect that has a CHANCE TO OCCUR based on ability coefficients and other descriptive factors. Life on Hit is such an effect. Stuns are another, there are many others. For instance: Bola Shot Thunderball can PROC Life on Hit with a coefficient of 0.500 and has a stun effect that will PROC 35% of the time ignoring all diminishing return effects for higher level play (Normal vs Inferno).
Life Steal has a 100% Chance to occur so long as damage is done. The big caveat to this is that the Life Steal ability has to be ACTIVE at the time the ability was used. When considering Life Steal on items, this is active 100% of the time. When you have no Life Steal on equipment, but use Shadow Power, the abilities used WHILE Shadow Power is active then regain life based on the damage performed by said abilities.
Abilities with delayed damage, such as Bola Shot or Spike Traps, as well as Chakram Suriken Cloud, Jagged Spikes, and slower moving abilities like Nether Tentacles, Ball Lightning, etc, will all regain life even after the Shadow Power has worn off of your Demon Hunter because those abilities were cast while it was active. This is called "snapshotting." They will also NOT regain you life if they were cast BEFORE Shadow Power was activated.
Here is an example:
you're fighting a Reflect mob, you're using Bola Shot
You fire off 3 shots and then realize, "Crap, it's a reflect mob." Turn on Shadow Power, keep firing Bola shots. the first 3 shots will explode reflecting the damage back to you, but will NOT regain you life, even though Shadow Power was activated at the time of the Bola Shot explosions. The subsequent shots will both reflect damage AND regain life, likely off-setting the damage done by the bola shots.
Let's say that you fired off a total of 20 additional Bola Shots while you were under the effect of Shadow Power. They will ALL give you back life from the damages performed by them. So let's say that Shadow Power wears off and you're out of discipline and cannot recast it.
You might notice you're STILL regaining life for an additional few seconds because your Bola Shots all had life steal on them from your Shadow Power.
I hope this somehow helps you, because I'm overcaffienated and just cant' sleep.