To increase your chances of survival you must practice only techniques that will work under these conditions and avoid techniques that will get you killed.
Maybe I am interpreting this statement incorrectly, if so let me know, but if I read you correctly you are telling people to learn how to shoot for combat by learning how to adapt to their instincts. I base my interpretation of what you said on the statement you made about what you believe would happen in combat:
Your body will almost immediately flood with aderaline as it prepares to fight or flee.
You will instinctivel crouch and attempt to square your body with the threat.
You will experience muscular contractions and lose the ability to make small. delicate movements properly.
Your eves will lock on the target in a form of tunnel vision
You will have to fight under this extreme stress. No one is immune.
I agree your body will become flooded with adrenaline. You will react in a certain way - possibly by instinct or startle response but also due to training, even some training you never realized you had. Television and movies show people reacting to combat all the time. You may react in some way you have seen repeatedly on the tube (quite likely not a good thing because Hollywood seldom gets it all right); or you may learn from a website like this and that can either help or hinder.
The important thing about training is not to learn how to react or act in line with any instincts you may have, but rather you need to learn what actions to take that are appropriately geared toward the situation in which you find yourself and quite possibly in opposition to what would probably be referred to as instinct. The thing is that you never have to reract in a so called instinctual manner, or even in a continued startle response manner. By this I mean for more than a split second. Sure we can all be startled, but we do not have to respond in a startled fashion more than a moment if trained properly, and sometimes we can learn how not to be startled at all. If you have trained for combat, you will likely react to a threat in the manner by which you were trained, and the more alert you are when the threat begins the faster you will revert to your training. This is not instinct at all, this is something you have learned from experience even if the experience was a training exercise. Heck even how alert you are at any given time may depend in good part upon training you have received on how to adjust you alert levels. Hopefully if your training was sufficient you will react in the proper way to survive and maybe even to win.
It is a fact that most of our fear reactions are learned and not instinctual. For instance: If you hand a baby or toddler a snake (and the child has never before experienced a snake or snake-like object directly or indirectly) the child will not show fear. If you hand that child a snake, and its mother screams as you do so, or if the child saw its mother negatively react to a snake in the past (without the child itself ever having before handled a snake) the child will most likely act according to the reaction of its mother. It will also react this way in the future to any further experiences with snakes. It has just learned via training from its mother to fear a snake. This fear can be unlearned, but that requires more training. The same holds true for most of our fear reactions, and to most of our reactions (not all of which will be fear reactions) in any shooting situation. So if we are trained well to act appropriately in a situation we may well survive it.
The truth of the matter is that people who are trained in combat or in tactical shooting react to threats as they were trained to do so in well over 90% of the shooting situations in which they find themselves (not necessarily the good training, so this is why the good training needs to be repeated over and over again). If you were trained to shoot a certain way and only in that way (not even necessarily in a tactical manner) then when you shoot at a bad guy you will likely resort to that same manner or style of shooting no matter how appropriate or inappropriate. If you are trained to shoot competively, and tactically (and by various tactical schools of thought or with various options) you would then have options to which you could resort and you could improvise. The thing with this is though, once you realize what is going on, you most likely will resort first to your training (bad or good - probably whichever took the best hold in you), until you thought process clicks in and before you would be able to impriovise if improvisation is actually needed. Of course if you were trained, among other things to improvise, then you will realize this option much more readily than someone who has not been trained to improvise.
In practical terms, and using some of your examples this works out as follows:
In combat situations you never need to develop tunnel vision. It can be avoided easily if you follow training that teaches you how to avoid it.
You do not need to go into an 'instictual' crouch if it is not what is warranted and if you have been trained as to what is the best position for a situation.
You will not necessarily fight or flee there is a third option and that is to freeze or stop. This is distinct from either of the other two and is a bonafide survival adaptation that can either work or get you killed you have to know when to use it or the others and training would be a big help there. Of course there is another option to combine actions - flee while fighting, and then to stop and assess.
One of the first things you should be taught about when learning combat or tactical shooting is to seek cover if at all possible when engaged. You can do lots of things from behind good cover, and maybe most importantly you can think more easily so as to assess the situation. This does not necessarily mean you will think of the right solution or that you will not panic - but the better you have been trained the better prepared you will be and the more likely that you will think of the right solution. Heck you can even be trained how to react if you start to panic or if fear seems to be overcoming you.
So in essence what I am saying is you need to learn how to act in a firefight, and you must also learn that you may need to improvise, and you must learn to think under stress or even learn that yes you CAN think under stress and you CAN TAKE PROPER ACTION under stress. This almost all comes about through training not through any instincts any of us may have.
All the best,