Chris,
It's not playing devil's advocate simply to ask legitimate questions, as yours are. RE: "or can the same principles used against a knife attack be used against a hard empty hand or blunt attack ... say from a Muay Thai guy or a boxer?". Yes, quite literally. I've done so personally with the occasional MT stylist and boxer at the MMA gym where I used to teach. Think of it this way, when you are evading, intercepting and redirecting an incoming blade, you are not making contact with the blade itself. You are redirecting the incoming weapon-bearing limb. If the person were to drop the blade for whatever reason before striking you, and bring that same striking motion in toward you, there is no net gain of mass or power on his part. In fact, there is a negligible net loss since the weapon's mass is no longer part of the equation.
You are still evading, intercepting and redirecting the incoming weapon-bearing limb, the only difference being that now the weapon is a clenched fist.
RE: "I knife only has to touch on the way past or back to have a serious effect.". Indeed. As I explain to all my students, unlike a club or similar impact weapon, the blade is a contact weapon. It need only come in touch contact with you to cause serious injury. For instance, I keep all my blades so sharp that often my larger blades will cut skin from the pressure of the weight of the blade alone. Not deep or anything, but enough to illustrate a point to some of my more serious trainees.
To further that point, if one is able to evade even the slightest contact from a blade and redirect that weapon-bearing limb, then doing the same against an empty-handed strike is just that much easier, if only for the fact that the consequences for slight touch contact are far more forgiving against a simple punch.
RE: "However, much of the knife work i have been exposed too has been to 'check' the knife arm. Not to let is continue on a path where it can change angle and cut you, or cut you on its current line, etc.". That's ok. I'm certainly not invalidating that approach carte blanche. Unlike a lot of my peers, I am somewhat less dogmatic about teaching bladed weapons tactics. There are a number of issues about which the knife community tends to become quite polarized. IME, I've met guys from every school of thought who could make what they preach work, even if it's diametrically opposed to what someone else is preaching.
To me, there are valid points to be made about all of them, and truth doesn't always require mutual exclusion. Being in a fight for your life against a knife is such a nightmarish goatf*ck of a situation that dogma won't much apply anyway, and it tends to blind us to the potential value of different tactical ideas.
RE: "Where as with a punch you can even let it move close to your body (even onto your body!) and use it or subtley re-direct it, with a knife you cant afford to do this.". Absolutely correct. This is why I don't train my people to purposefully allow an incoming strike to contact their bodies in order to stuff it or deflect it. Is it a valid tactic? Very much so....I myself can do it to the point of being able to injure the incoming punch in about 1/3 to 1/2 the attempts if I'm really interested in harming my opponent. I've done a lot of training in that regard. It's great against an empty attack, and frankly, sometimes you just don't have a choice because if you get into a fight, there's a good chance you're going to get hit in some way.
However, because of the inherent and irretrievable risks that a weapon poses against that tactic, I teach my guys not to do that, but to deal with it with your limbs if at all possible. Now, before some readers might dismiss that by saying, "Well, that's just Chris....he's dealing with high-speed operators, I don't need to worry about that risk", I should mention that I teach everybody that idea. Average ordinary citizens just trying to get home safely are at a similar risk of a weapon being present if they find themselves in a real life-threatening assault.
If you're talking about puffing up and getting into male dominance fights at the local honkey tonk or trendy dance club, that may not be the case, but then you're on your own. Those fights are almost 100% avoidable and if my students find themselves facing those types of situations often, then I have no sympathy for them and feel that they get what they deserve. I train people not for chest thumping against the local stable of gelled-up satin-shirted fratboys at the club, but for situations in which their life may be threatened and they are not able to avoid it. For such situations, the chances are that there will be at least one weapon involved, and they must train accordingly if they want to be prepared for it.