All unique monsters/groups of monsters will heal when there are no players within their range.
The only time they do not heal is when engaged in combat.
As for the rate, I am unsure, although it may be slightly higher in the latter difficulties (I can do some testing when I get home from work).
Enemies tend to stand around in the line of fire for traps in many cases, especially in the early going. The issue is more one of triggering them while they're nearby. For instance, in the Cathedral there are many, many, many chandeliers. Typically there will be "zombie type" enemies feasting on a corpse directly underneath. The trick to these tends to be just getting to the chandelier chain and activating it without disturbing the zombies. In some cases, it's not easy or worth it to trigger them - the enemies are between you and the chain, and you might as well kill them while you're headed in that direction.
Many of the other traps in the game are actually destruction rather than interaction based - so if you happen to be playing a barbarian or monk (with limited ranged skills at low levels), you may not have noticed the difference. These traps are generally pretty easy to trigger before the enemies notice you, and they will severely injure or kill enemies, at least in Normal. You can tell the difference in a class of traps by what your character does when you interact - if they swing their weapon, it's a destruction-based trap, and if they don't, it's interaction based. Oak also points out that the color is different - interactive is blue, and destruction is red.
Unless you're farming trap kills, it's typically not worth the trouble to go and kite distant enemies into the trap kill zone - you can probably kill them faster than it would take to get them to follow you anyhow.
At higher difficulties, while the trap damage scales, it does not scale as fast as monster health does. Therefore, getting "pulverized" bonuses is much harder, as is the related achievement. If you're trying to farm this, I'd probably suggest running the Cathedral on Normal, with no followers or pets.
Best Answer
If you are looking for good old hack n slash action, wait until you start running Nephalem Rifts which are designed exaclty for that purpose. Even though the number of enemy is not unlimited the density is really high and once you've complete a Nephalem Rift you can directly jump to another one and start slaying hundreds of mobs again.
If we are talking about infinite spawning mobs late in the game there is one location in act V that is pretty famous. Just follow those steps :
Note that this method has been nerfed, the monsters still spawn but no longer gives XP or loot.
Here's a short video I could find on youtube demonstrating this exact method :