Plus or minus a few degrees, not much of a stretch really. Oh and the fingers always make some contact, we human beings compress and deform a bit when we get hit.
Actually, it does make a difference... 90 degrees means that, as the knuckles impact, so will the front of the fingers, which can force them back, and damage them. Less than 90 degrees, and the impact is with the fore-knuckles, which take the brunt of the force. Yes, the fingers will still contact, but they don't "impact" (take the major force of the strike). Again, the words are important.
Naturally, I am only talking about "regular" fists here, rather than any specialized ones... if I was, I'd talk about many, many different forms, ranging from striking with the first knuckles of the fingers (a "panther fist", or, in my systems, shikan ken), a single extended first-knuckle strike, a strike with the thumb, the first knuckle of the thumb, even up to a fist found in Yagyu Shingan Ryu, where the hand is held by keeping the tips of the fingers and thumb together, holding the hand "as if holding an egg", meaning that the fist is never closed... and the striking method used in this form is to swing the arms in large movements. After a few years of training like that, you should be able to break rocks with this "unclenched" fist. But, as I said, I'm only referring to a regular fist at the moment...
Any reasonable amount of stress on the bones will strengthen them (Wolfe's law, cortical remodeling), a single punch through 2 or 3 boards will do more to condition the hands than a single punch on a punching bag. Stronger fingers mean that you can clench the fist tighter.
No.
Cortical remodeling, which is part of Wolff's Law, refers to the effect of striking an object over and over... such as makiwara or bag work. Knuckle push-ups don't have the same effect, as they don't give the same stimulus. They'll help toughen up the skin on your knuckles more than anything else. Oh, and Wolff's Law really deals more with the effect on bone mass that muscular stress (loading) has. By building up the muscular groups attached to the bones, it forces them to become denser, or grow. Knuckle push-ups don't provide that stimulus. Striking through boards doesn't actually do it either, as you're not generating impact to create the effect of cortical remodeling, unless the boards don't break. Really, bag work and makiwara (the same thing, really, just using different equipment) is the way to employ the effects of Wolff's Law, as well as condition your hands for striking. The rest you suggested isn't.
You're right I did need more training 20 years ago.
You're kidding? You spoke about "other injuries" by citing ones you've sustained, which all showed that you were striking incorrectly (missing the bag, striking with the wrong part of your hand etc), and when called on the fact that your story didn't show an issue with the striking ideas, but with your execution of it, you come back with "you're right, I did need more training 20 years ago"? Really? You injured yourself as a beginner two decades ago, and think that's relevant to whether or not the strike is dangerous?
If you strike badly, whether due to inexperience or poor technique, you can injure yourself. That's really all you showed. Nothing about whether or not the actual method is flawed.
You seem to believe that people's heads are immovable objects that people can't punch without breaking their hands, palm strikes can not injure the striker, people are powerless against the influence of adrenaline and people always go on "unconscious autopilot" when attacked and you say my knowledge of striking needs work.
You really seem to have missed, not only what I've been saying, but, well, reality on a number of levels here. Badly.
This part I agree with you - I can think of no good reason to stomp on someones head. One question though - how do you normally stomp on the wrists of the knife holder, is it after the knife has been removed or while they are still holding it? Just curious.
That depends entirely on the situation. But it does rely on them being taken down already, and I wouldn't suggest stomping a wrist that is holding a knife... there, I'd pick another target.