Dear Lele -
If only we were closer (geographically) as I would love for you to visit my shack and I could demonstrate why the RST system was designed as it was and, at the time, specifically targeted to CW (as that was the predominant mode of course).
You are very close with your point here - "...but I never found in 14 years of Log4OM a list of valid values." That should tell you something, at any particular time - *any* of the RST values might be the one you send (and all of them can be valid).
Strong signal but almost impossible to copy due to QRN (like lightning crashes on 160m) - RST 399 wid QRN - Very weak signal but good copy with auroral flutter (very common on the long bands this week due to solar flares) - RST 529A.
How about listening to a "boat anchor" (old tube-based rig) that has significant "Chirp" - RST 559C.
Look at this picture from the 1969 ARRL Operator's Manual - page 23 discussing the "Tone" attribute of the RST. (Also remember that almost all rigs, at that time, were tube based and many were homebrew transmitters.) See how the sentence that starts "Note that a signal can easily be T5XK..." describes why a list *couldn't* be fashioned to account for all valid combinations. (Well, I guess it could, get out your calculator to figure how many permutations exist. hihi)

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Anyhow, I hope that you will find a CW operator that you can trust somewhere close to you and just spend a bit of time with them. (Note that during contests you will almost always hear 599 as the RST but it doesn't reflect conversational CW - a mode in which we want to know what our signal sounds like at the receiving point.)
If you ever want to hear what RST really means to an operator, I would be happy to email you some audio (MP3) files of typical QSOs that I have. Many of them exhibit the auroral flutter, or other artifacts that the RST code was developed to describe for CW operators.
I realize that you can't get a good picture of what I am describing without the experience and only sorry that I can't really show you.
Best 73 from New Hampshire -
Larry N1FG