Immolation trap is had MUCH earlier than snake (lvl 66 I believe). Snake is something of an end-game trap.
Immolation works against almost every target in the game, only a handful of mobs and bosses are immune to it.
Immolation is both a planned and opportunity trap. Some traps are CC and safety, end-game wise, immolation is just a free damage add when other traps are pretty much useless.
Proper trapping in a group is not really done the way you describe. Planned trapping is usually done during the pull phase. If CC is not an issue and you want some additional damage, you can drop an immolation trap down, HOWEVER, many groups will not allow this as it generates aggro on you and not the tank, I heard a rumor that you can misdirect trap damage onto the tank but there are far better shots to use to do that with. So essentially it's counter productive most times in a group. In combat, it's an opportunity trap you dump on a target to help burn them down. Useful in a pinch if they can't be CC'd.
Solo use of this trap is easily done. You can do it how you describe which is basically a pet pull. I still use that pull in end-game when I'm solo. You can also set up an immolation trap for a patrol you know their route on. It's a 'trap pull' (to give it a name) and it's really only useful if you want to pull a pat from between two mobs that aren't close enough to aggro on you but you want that pat to run for you when they hit 'spot X'. This is normally done with a well timed shot but if you don't want any mistakes and know the pat route, you can use a trap pull. It was fun to do. You can't use snake as they can go for the mobs, aggroing them, so immolation works well.
I still use explosive trap from time to time. Great for AoE.
BTW, the stickies are awesome to learn the basics and some more advanced tips and tricks. People on this forum are generally very helpful and will even answer questions that are in the stickies. Sometimes, folks can be in a bad mood and give a sharp answer. It's not good, but it is life. It's hard enough being a hunter, we are surrounded by many players that actively try to make us look like idiots. When you find someone like that posting, it takes serious control not to lash back out at them. I'm just one of those that was a total idiot and now can probably claim minor stupidity from time to time. I guess that makes me an evolved idiot. I'm only less of an idiot because of the very people on this forum that may seem like hardasses but in truth, they do always help.