Good question. Yes, it’s possible to undertrain relative to current fitness levels, but it’s easier to correct undertraining than overtraining. In general, it’s best to start conservatively, and then increase if appropriate.
A big mental adjustment that I had to make when I moved from active-but-unstructured to structured training is that although I may have trained similar volumes in general, the chronic nature of structured training brings a unique (and necessary) type of fatigue.
The chronic stimulus is necessary for ongoing adaptation, but it can be deceptively easy for days and weeks until a big enough load adds up.
I’m afraid I don’t understand the rest of your question. Can you clarify the following?
…the 200-250 annual hours estimates yield individual, non-strength workouts shorter than an hour until well into the Base period, with “remaining Zn1 volume” larger than the longest workout recommended.
Are you asking if the 200-250 hours isn’t enough volume? Or if a workout shorter than an hour is too short? Or if the remaining hours in Zone 1 have to be done in one workout? Or all of the above?