Like Medulant, I contend/accept that the ~translation~ from teacher to student perhaps was poor? Possibly within our lifetime that became a literal factor, ie "we" did not speak their language at all sufficently/at-all in order to get that level of understanding possibly?
Physically I can practice and eventually uncover whispers, things hidden completely in plain sight. Yet a few well choosen and specific words might open new perceptions, an awareness which once opened could produce such epiphanies <sp.?> far sooner!?
<<how much 'fixed' applications are taught in your dojo
There are certain structural mechanics which cannot be ignored, within all small movement patterns of a kata. If my arms are medium/high it is extremely unlikely (merely for a random example) that I am a-typically coping with your legs kicking my head....
Unless my body is structurally positioned to withstand that weight, which is why very specific stances and not others are utilized. If my arms and legs adhear to the correct starting and ending positions (ie without alteration) within a sequence of movements, therefore are identical to the empty handed form... anything theoretically would be potentially fair game.
Hense the "WHY" it is a lifetime study...
As for intensity, you need to start slowly, very slowly so that the mechanics can be studied. If I go maximum intensity from the very first moment I enter a class ever, the issue becomes simple instinctual survival NOT technical improvement and exploration, honing....
Intensity is fine, provided both/all partners verbally express that wish AND, and know each other well enough so that even "hard" nobody goes primal ~red zone~ and looses their control. This is called practice for a reason.
<<Secondly, re-engineering of application is widespread in Karate practise, why is this?
Incorrect perception by teacher or student would be one of my more favorite hypothesis. Incorrect transfer for any number of reasons. Shallow comprehension. Any of these or a thousand others might explain the ~negative side~ of the proverbial equasion? On the positive, there are so many fascinating potential and completely viable possibilities for tiny movements that require temporary shelving at some point to eventually re-explore. Otherwise literal insanity sets in...
Knowing the "original", frankly I do not know is necessary. It would mean, in some ways that having only "one" answer would become in some respects possibly a dead thing; kept from the past without understanding any manner of WHY concerning those movement patterns? Merely cookie cutter copying... from one ~generation~ to the next
Consider, I create and invent something, tell you and DEMAND you never, ever alter, change any facet of it ever. Simply do it, over and over, and over. All fine and good, but what happens when I am a different sex, very different mass, radically different age, anthetically different temperment, etc., etc. If, I am restricted to merely copy the movements of someone who was thirty years younger, an angry person, obscenely powerful and I am none of those things.... ONLY maintaining their practice would mean I am copying it, not understanding what they were telling us
In this regard reverse engineering is obscenely correct & dare I say required/mandatory.
<<Why didn't/don't many Sensei pass on/know the origional applications from history?
Well human lifespan comes to mind... if I were a able to be say 150 years old, or 200, I am sure my art and the practices I explore might look radically different in some respects. If I could be, several hundreds of years old... I am confidant I would still have dozens of nuances and subtleties for every single movement.... I would have had lots of people to practice with/upon!!!!
This how it is meant to be...
Now, I accept our practices fundamentally are about maintaining our personal boundries against someone(s) who cannot/will not be convinced otherwise. Now having said that, I am not fundamentally compelled that they were supposed to be USED. Learn, understand how to produce the maximal power we are capable and learn how to not get drawn into others stupidity to me sounds like ancient core wisdom!!!
Merely my perspective, hopefully coherent... I could surely be mistaken,