I have based my interpretation on a few assumptions. 

I.33 describes a functional martial art

This is what I just expect. Some people in the past came with idea that I.33 is just a training manual (about how to teach lessons in martial arts) and thus it can portray actions that don't work, because they are just training drills. I don't like this approach, because even a training for combat should be a reflection of combat itself. If you train a situation and its solution, it should be the same situation you can get in combat, and a solution that would work in combat. Otherwise it wouldn't be a useful training. 

That said, I am also accepting the fact that the manual is organized in the same way the priest was teaching, and that it might be portrayed with some compromises regarding safety in training. That is why, for example, in I.33 we never see one fencer hit the other - they always aim next to the opponent, in order not to harm him.

I.33 describes a well defined system with inner logic and consistency

I assume this on the fact that the priest was a teacher, and thus had to have a well organized method to convey information to the students. If he was just a warrior, he could be winning just by intuitively doing the correct move, without any system behind it. But the priest had to explain mechanisms of combat to students who didn't know much about it. For that you need a logical and understandable system, which is what i believe I.33 contains.

The authors of I.33 were trying their best to give us correct information, within the limitations they had

This is a neccessary assumption to even get anywhere with the interpretation - I have to assume that the authors of neither images nor text were intentionally trying to deceive the reader. Apart from that, I also assume they were not lazy and really wanted to pass the most information they can to the future generations. 

But they were working within limitations, and that is why I.33 can be so incomprehensible. These limitations were for example kind of an unrealistic graphical style, or limited space for text on quite expensive pages of the book. 

Interpretation

When I am creating my interpretation, I am always working in two steps:

  1. Generate an idea.
  2. Test if it is plausible. 

How you generate the idea is not important - it doesn't matter if you got the idea by carefully looking for similarities in other martial arts, or by asking demons summoned in a dark ritual. The idea itself is the only thing that matters.

What is important is how you determine if idea should be accepted into interpretation or not. This is the way that I was using.

Test if it works in sparring (if applicable)

This is an important test because of my assumption that I.33 presents us with a functional martial art. But it is also tricky, because

  • not everything can be tested in sparring. How would you test an idea "I think that schutzen is a parry" in this way? It is about meaning of the term, not about a technique itself.
  • if you pull it off in sparring, it may be just because the opponent had a bad day or was too inexperienced.
  • if you can't pull it off in sparring, it may be because your opponent was always able to counter it. Which is absolutely OK, because every technique in every martial art has a counter.

So this type of test needs a bit of experience in other martial arts, understanding of timing etc., in order to see how plausible the technique itself is. What you are looking for is not a technique that always succeeds (because no such technique exists), but a technique that has a reasonable chance of succeeding under pressure and in full speed.

Test if it matches the images and text (if applicable)

The manual is possibly showing us something relevant in images, and says something about it in the text. Do the images fit my idea? Can the text be understood in a way that matches my idea? This is about using the the assumption of authors doing their best to give us correct information.  

And it is also a bit tricky, because both images and text can be deceiving, even if we assume that they are not deceiving us intentionally. On the other hand, I believe that the images follow pretty strict rules and it is possible to determine what real-life situation they are portraying.

Test if it matches a common sense

This is using the assumption of inner logic and consistency - the system of the priest has to make sense. Terms used should make sense, there should be a reason why the term is what it is. For example, a statement "schutzen is everything you do" will pass all the above tests, but not this one. Because the definition, that allows everything, is not useful, and the teacher wouldn't use it.

Test if it is consistent with the interpretation so far

This is extremely important, again because of the assumption of inner logic and consistency. Every single technique, every image, every term in I.33 may be interpreted in many different ways that will pass all the tests above. But there will be only one consistent intepretation of the manual as a whole. 

If I find that a new piece of interpretation is not consistent with what I already have, I am looking for reasons. Maybe there is a way how to modify this new interpretation in order to still pass all the tests and fit. Or maybe there is a way to modify existing interpretations, so that the new one will fit and it all remains consistent. If neither is possible, then I can't accept the new interpretation and I have to search on.