Rhythm & Meter - some questions

[pianovirus]

I have a number of questions that keep confusing me. Even if you answer only to one aspect it would be helpful. The topic is rhythm and meter, in the following sense:
- rhythm = the true patterns of note durations
- meter = our perception and anticipation of such patterns

1.) In meters like 5/8, 11/16 etc., it seems to me that we always hear the meters subdivided into smaller groups of 2 and 3. In other words, I think there can be no meter that contains 2 or more consecutive units of "equally unaccentuated" notes. That is, in any group of four or more units, we always perceive at least one (or more) "secondary" accents besides the main accent. Any opinions on this - do others agree?

So the straightforward way we would perceive a 5/8 is as 2/8+3/8 or 3/8+2/8.

Above we talked about sequences of unaccentuated units (and as I said it's always either one or two units until the next primary/secondary accentuation). Now - can we perceive sequences of accentuated notes as well? For example, could we play and perceive a repetitive 5/8 meter as follows (+=accented, -=unaccented):
+-+-+ | +-+-+| +-+-+ (2/8+2/8+1/8).

2.) Messiaen's (and others') "added values":
- How do you practice exact execution of such passages?
- How do you count and what do you feel in terms of rhythm and meter?
- It seems to me these added values break the perception of a meter to the extent that no meter is perceived by a listener since it is constantly changing?

Brief explanation of added values:
After or in the middle of a "regular" group of note durations, a note of smaller duration is added (here a 16th):

Here is a longer example, with + denoting the added values. Note that without these values most measures would be naturally seen as 4/4 etc., but now they are 4/4+1/16 etc.


3.) Finally, what is the meaning of bar lines in music that is notated without a meter?
Is it just a sign of where the primary accent in a group of notes should fall?
Is this the way in which the unsynchronized bar lines in Ligeti's Desordre should be understood?
To someone who has played this, what is your perception of rhythm/meter as you practice or perform this?
https://www.bernardhughes.co.uk/assets/pdfs/ligeti_analysis.pdf

[ted]

All this assumes that rhythm is homomorphic to some sort of discrete visual representation.  For me it is not. Rhythm for me comes close to being the intangible soul of music. Even in classical music performance it is what distinguishes the brilliant from the technically accomplished but prosaic. It is what separates a Waller or a David Roberts from his imitators.

Even aside from this general unsolvability, there remains the fact, uncomfortable for many, that something can be written or intended in one metrical sense and be perceived, quite validly, in another. I recall a delightful "waltz" by Scott Kirby on a CD of Terra Verde, or ragtime derived piano music. For the life of me, I cannot naturally hear it as anything other than 6/8. By a deliberate shift of perception I can hear it in 3/4, but it loses its charm somewhat. It is rather like looking at one of those isometric illusions which have more than one state.

I am just throwing in a couple of initial thoughts here, in anticipation of a long and profitable discussion, as this is a really good topic, which goes right to the core of how we respond to musical sound.