Well you are right that the piece is not entirely modal! Occasionally there are some notes that not part of the given mode or scale : )
Ok i'll try to give a little bit of context here, because your confusion is actually perfectly valid:
You can see in my name the word "MITA" that stands for "Music Interval Theory Academy" which teaches a system similar to Spud Murphy's EIS (Equal Interval System) method.
(First of all I want to make clear that I DO NOT COME HERE TO ADVERTISE ANY ORGANIZATION OR SYSTEM! I am just trying to give a little bit of context how this piece came to be!
So I hope this post and thread will not banned to infinity by the mods just because i mentioned this)
The point is both systems have a kind of intervallic aproach to music composition meaning they focus more on the distance between different notes and intervallic formulas instead of thinking in strict tonal centers and scale degrees while also teaching a mindset and giving tools that actually generate music. There are some free courses online you can check it out if you interested and also some YT videos explaining the basics...
Now more about the guidelines:
So this was originally an assignment about Combining Modes using a set of rules. It goes like this:
1. First write the melody for the soprano part from bar 1 to 16 (line writing) paying attention to choose every second bar a new tonal center and a new mode!
2. Next write a counter melody in the tenor part (without thinking in vertically or in chord structures (!) only horizontally!) doing the same as the soprano choosing every two bar a new tonal center and mode (different from the soprano part)!
3. Now write the alto part using the same modes/tonic as the soprano
4. Do the same with the bass using the same modes/tonic as the tenor
5. Profit (???)
Now how i applied the concepts?
1. First i came up with a LOGIC how i will choose my tonal centers and scales:
1a. Tonal centers:
- In the Soprano/Alto: Mostly following the Circle of Fifth (It's called a Root Cycle 5 in MITA or just RC5) except the last two bars: D - A - E - B - F# - C# - Ab - D(!) - A(!)
- In the Tenor/Bass: It's more complicated, but basically its a combination of different RCs (there are more than just your basic Circle of Fifth - actually there are 6 different RCs in an octave that you can combine freely)
1b. Modes: Random (but paying attention to always choose one not used before and use only scales with 7 notes)
2. Second i wrote the melody first in the soprano paying attention to all the mode changes, then wrote all the other parts afterwards
Now here comes the source of your confusion basically (at least what i assume):
- The melody is not your usual 8 bar period with two half periods, so it seems like it doesn't have a structure (even though it literally has)
- The scale/mode changing picked randomly which result in not the smoothest transitions possible (if someone wants to achieve that one can choose neighbouring scales that's close together only differing in 1 scale tone at once - like Ionian/Lydian etc.)
- Third source of confusion come from that on top of that i added some out of scale notes sparingly (that adds to the more cromatic/chaotic feeling to it, which is a nice technique to hide or mask your original ideas/scales, but also it can confuse the hell out of people in some cases i guess)
- Also in the final score I rearranged the the whole score to more fit to the tempo changes i applied to them in my DAW (because originally i felt some notes should held longer in the melody, or should be quicker, I marked these places with fermatas)
This results in what you mentioned that at the same bar there is a C# and a C which originally occured in two different bars.
All in all i can easily understand why you felt confused, and thanks for your clarification also!
See the attached original sketch i made the final score from posted above, maybe it can clear up some things.
(Also I'm still learning to use this technique btw, this was my first attempt, never wrote polytonal/polymodal pieces before, but this can be a very powerful tool to came up with interesting new musical ideas for writing freely while applying logic and rules at the same time that guide you in the process)
I know it sounds confusing so, if someone has a question just ask!