1. Cover garments don't have to be a vest, unless one so chooses to wear it at the match since they choose to wear it daily anyways and that's what they want to practice with. The IDPA rule only requires that it be concealing the holstered gun when you lift your arms straight to the sides up to shoulder height. I sometimes choose to wear the vest, but I generally am wearing a shirt that I wear most everyday. At one of the matches last year, I even chose to wear a clinic lab jacket since that is what I sometimes wear at my private practice office. I've even worn my winter jacket. Granted, a couple of those clothing items may add a slight bit of time to my draw, but my approach to why I participate in IDPA may be different than others....but that is a whole other discussion. 
For me, the "vest" part is simply based on the fact that since I can't use my normal carry holster, I'm not going to pay a bunch of money for another quality CCW holster that I'll never use outside of IDPA (traditional IWB prints on me in a stupidly silly fashion, and IDPA requires the holster to be at the side of the body at minimum)---so I'm going to pull out a basic range holster that I already own, which happens to fit the IDPA criteria.
And because I'm using that basic range holster, I'm not going to simply wear a shirt over it, because it prints ridiculously. If I was wearing my carry holster, I'd simply wear what I normally wear every day. But since I'm not, to conceal this holster I have to wear a significantly more voluminous cover garment.
Given that, and since I'll be shooting IDPA in all sorts of weather, it is easiest to pull out a "shoot me first" vest, since it will work in all sorts of weather and clothing, and fulfills the IDPA cover requirement, and I don't have to buy any more gear. And since this is a game (to my way of thinking, and opinions vary), I might as well use equipment that optimizes my chances of winning.
Can't use actual carry gear + not buying any more equipment + increased chances of winning -- makes my choice pretty simple.
2. The IWB appendix carry not being allowed by IDPA is purely for safety reasons, although bullit and I are always "happy" to see participants at IDPA Nebraska matches 
. Hopefully, folks who do elect to carry as such are careful in their draw as well as reholster so as not to blow off their privates with a negligent discharge 
, but such situation would also quickly have adverse ramifications for the host range as well as for the continuance of the IDPA sport at the host range. 
I'm thinking that given the large number of people who carry AIWB, and the fact that we don't hear about people shooting off parts of themselves, I'm thinking that potential safety issues are overblown. That isn't to say that I haven't seen plenty of people who are scary enough
already with basic OWB hip holsters---merely that the people who carry AIWB don't seem to be shooting themselves, so perhaps the risk isn't any greater than the risk of people shooting themselves in the leg while drawing or holstering IWB.
However, I understand the thought process, and I understand why IDPA doesn't want to go there. (Amusingly enough, you can run your AIWB rig from concealment in USPSA if you want to! Matter of fact, I know someone who does precisely that, though he doesn't shoot around here.) I disagree with IDPA's opinion, but I understand why they think that way.
Regarding tac lights: Yes, I normally do have a lanyard on my tac light. At the moment I don't, however, because it finally broke last week when I was practicing with it. So at the moment, I'm lanyard-less.
Carry position: pants pocket (I'm normally wearing a version of cargo pants, and there is a front pocket perfect for it).
Lanyard use: Never would put it around my neck. (That wouldn't even occur to me.) Have put it around my wrist a couple of times, given situations in which I was going from a light situation to a dark one in which the "dark" area was large enough that I was going to be in it for awhile. (If the "dark" part isn't that big, I'm playing the odds that with 17+1 rounds to use before I need to reload, I can get back to the light before I need that extra hand back.)
Practice: Yes, I do. In both dryfire and live fire---which is why I ALSO know that my one-handed shooting using a light (especially in dim light situations) is both slower and less accurate than two-handed shooting using a weapon-mounted light, in particular the weapon-mounted light-laser combo I currently use. (I note that I don't have that on my carry gun, that is the home gun. And lights and/versus lasers is a completely 'nother topic.)
Regarding use of lanyards and such in IDPA: My point is that for a significant number of the situations I can think of in which I'd use a handheld light, I _would_ have already placed it in my hand, and thus would have put the lanyard on. If I'm taking a long walk to my car in a parking lot that is dimly light, I know this ahead of time.
Sure, if I'm suddenly surprised when the lights go off, I wouldn't have the lanyard on---but then again, I wouldn't have the light in my hand, either. And if I'm going to be attacked in 2 seconds, I'm going for my gun and moving fast, not worrying about the light.
But in cases where I had the light in my hand, I can't think of a whole lot of situations that are realistic for
my lifestyle (as such, results may vary!) in which I wouldn't have had time to stick my hand through the lanyard first. Maybe I'm not thinking of the right type of situations...?
Regarding a tug-of-war with the light/lanyard: Most handheld lights aren't big. I have a difficult time thinking of anyone trying to take it out of my hand. And tac light lanyards don't have much extra cord, so it isn't like there is going to be a loop for them to grab. That being said, if they want to tie up one of their hands attempting to grab an object that is fixed to me, they are welcome to go ahead.
For me though, this is a purely academic argument---mostly based on the number of people who keep telling me that IDPA is "realistic self defense training." If you think of IDPA as a shooting sport (which I do), these issues aren't important. It is only if you think of IDPA as "realistic training for self-defense" that a number of these issues crop up.
Regarding O rings and such on handheld lights---you know, when I was at the Rogers Shooting School, he showed us the Rogers method, which is really sweet if you have the right kind of tac light and an o-ring attached to it in the right way. It is a great method that enables you to have a light and still shoot two-handed with almost the same level of speed and accuracy as normal shooting. It ALSO takes a very large amount of time and practice to make it work, especially under stress. Since I had never tried it before, I completely sucked at it.
...I was about to go on about WML and handhelds, then I realized that really was a very different conversation. So I'm going to start a thread about that in the discussion section elsewhere, so as not to derail this thread further.
Bullit: didn't EVERYONE argue when playing D&D?
I note that arguing is actually a good sign, really. It means that people are
thinking about what is going on. When no one is arguing or discussing, either people aren't thinking, OR things are so bad that no one wants to talk about it.
And healthy discussion means that people often have to support their opinions with facts, which means that sometimes, people learn something. Or at least get exposed to ideas different than their own!
Granted, it is almost impossible to get a gamer to start thinking realistically....
