This is a misconfiguration in the ubuntu distribution that leads to inconsistencies in switching keyboard layouts when mozc is involved, not a mozc bug.
Mozc just does what it's been told to.
The main point is:
"I don't understand the rationale behind the "Japanese" (no mozc) keyboard layout switching to jp (ie: actually doing setxkbmap jp) but "Japanese (Mozc)" not doing it."
Let me rephrase this. This is not a bug.
This is a misconfiguration in the ubuntu distribution that leads to inconsistencies in switching keyboard layouts when mozc is involved, not a mozc bug.
Mozc just does what it's been told to.
The main point is:
"I don't understand the rationale behind the "Japanese" (no mozc) keyboard layout switching to jp (ie: actually doing setxkbmap jp) but "Japanese (Mozc)" not doing it."