If it withdraws into an enemy unit's location it is eliminated (A11.21), however since the Japanese are in different locations by virtue of being in PBs
This is what we thought, as well. However, there is the question of whether CC is mandatory or not. My understanding is that a unit sitting on top of a pillbox doesn't
have to CC units inside the pillbox, but while that makes sense, it's a little less clear for a unit underneath. They are in separate locations, so we decided it was optional.
But afterwards, what pillbox TEM (if any) would apply for attacks coming from an attached trench? One would think that IRL, where the pillbox opens up into the trench wouldn't be as hardened, and so more vulnerable to attacks.
And does having an enemy unit in the trench of a Japanese bunker affect the ability to use an attached tunnel? We decided "no".
I think he would just remain outside the PB location (no 2nd CC at this point).
It ended up being quite entertaining
I withdrew into the second trench, with the possibility of CC. We both decided to attack, with no possibility of ambush (since there was no Advance). Somehow, my HS managed to survive, and since there was no Melee, he cycled in some new concealed Japanese units next turn and killed it. But for all the messing with his head dealing with it, I definitely got my money's worth