You mention 1.) realism, and 2.) ease of adjudication. I'll address your comments in this order.
RE: realism, yes, people fire blind in combat, but as far as I can tell don't fire blindly at coordinates, they fire blindly at certain vectors: "up this corridor" and not "at a five-foot square 45 feet up this corridor." If there's an enemy in the dark 55 feet up the corridor instead, guess what? Your arrow aimed at 45 feet away will probably hit him. Ergo, having to guess the exact five-foot square that someone is in is unrealistically harsh, and also gamey--unless you're firing at someone so far away that projectile drop is a real factor. (D&D tends to neglect projectile drop anyway. Who says "You can't fire that longbow indoors because it will hit the ceiling"?)
RE: ease of adjudication, "obscurement is usually in tight little zones." In my experience, obscurement comes in huge swathes. When fighting at night or underground, everything not within 40' of a torch or 60' of a character with darkvision is heavily obscured unless there is a particularly brilliant moon. If I have four PCs and eight monsters in combat, trying to track the location of all twelve characters without showing any of them on the battlefield except the ones carrying torches/within darkvision range of each other would be an amazing amount of hassle. Even if I'm using a grid as I sometimes do, it would do me very little good because to keep the game fair I couldn't actually show anything on the grid, and I'd have to have my players keep their coordinates secret as well.
N.b. under the "can't hit me unless you guess where I am" rules, the winning strategy is have a high-AC guy like a monk or paladin carry a torch around to follow the bad guys while using his action to Dodge while the other PCs pelt the bad guys with advantage from outside the light radius. Result: the entire party basically gets advantage to attack and the enemy has to attack your toughest target at disadvantage. This does rely on a saner interpretation than RAW of what "heavy obscurement" means, since RAW (PHB 183) says that you can't actually see a guy holding the torch unless you're within the torch's light radius, which is bonkers and can't possibly be RAI.
You are overcomplicating the situation. I've thrown RAW, RAI, and that trash from 3E out the window. Never, ever liked that rubbish from 3E. Never heard either term prior to 3E. Don't intend to listen to much of it in 5E. I hope by the end of 5E's run, those terms are dead in the water. I run things by what makes sense using the simple rules system in place like I did in editions prior to 3E. Rule lawyers can go play some other game as far as I'm concerned. Worst part of 3E was rule lawyers trying to hold DM's hostage to problematic rule interpretations. No prior edition to 3E had ever allowed such a thing. The overly large rule set in 3E allowed that type of player to thrive. Glad it's gone.
In my experience, obscurement comes in small areas like tunnels or rooms underground or inside a structure, clouds, small areas of darkness, and the like. You must fight outside an awful lot to have your common experience be large areas of obscurement. Creatures with darkvision don't worry about light too much, so obscurement would only matter if you had a group with a darkvision advantage. As far as your crazy scenario with the guy running around with the torch, I guess that would work in very specific conditions outside with a wide unobstructed area. It wouldn't work too well in dungeon tunnels or rooms since an opponents darkvision would extend that far or dense forests or swampy regions where obstacles obscuring sight would create problems. Though it wouldn't be unheard of as keeping light on enemies while denying them the ability to see you has always been a desirable combat advantage.
We tend to keep it simple using the following guidelines:
1. A PC can in general fire at the last known location of an enemy.
2. If you choose to fire blind while following the direction of someone shouting that can see, you can generally target the correct square. For example, if the warlock with Devil Sight is fighting opponents whose last known location you know of is shouting where they moved, you can take a shot.
3. If you fire blind without any awareness, you risk hitting the PCs if they are in the path of fire. You will make a hit roll against any target in the path of the projectile doing damage to the first target you hit.
We keep it simple adding enough verisimilitude to provide a sense of the danger of firing blind. We don't want the situation to become overly complicated to the point it slows down play. I don't worry about realism so much as verisimilitude. If I worried about realism, I wouldn't be letting longbow users fire two or three times in six seconds. Many weapons would do next to nothing against plate armor. And trolls and dragons wouldn't exist. It's always a matter of maintaining some semblance of verisimilitude to maintain the suspension of disbelief as much as possible. We feel firing blind should have some consequences and came up with simple guidelines to follow to make it at least slightly dangerous.
We're a group that enjoys discussing what we think might happen in a given situation whether based on real world or cinematic experience...as long as it doesn't slow the game down too much.