Page 13-14 Europe second ed. manual:
Sea Units Starting in Hostile Sea Zones
At the beginning of the Combat Move phase, you might
already have sea units (and air units on carriers) in spaces
containing enemy units that were there at the start of
your turn. For example, an enemy might have built new
surface warships in a sea zone where you have sea units.
When your turn comes around again, you are sharing that
sea zone with enemy forces.
Sea Movement Example: Sea units other than
submarines must end their movement when they enter
a hostile sea zone. The destroyer must stop in this zone.
However, the submarine can pass through safely because
the enemy unit isn’t a destroyer.
Amphibious Assault Example: Amphibious assault
movement occurs during the Combat Move phase. In
this example, the U.S. battleship must destroy the enemy
destroyer in the sea combat in order to clear the hostile
sea zone so that the transport can offload into Normandy/
Bordeaux. If there had been only defending submarines
and/or transports, the attacking U.S. player could have
ignored those units, or could have conducted sea combat.
If you are sharing a sea zone with surface warships (not
submarines and/or transports) belonging to a power with
which you are at war, this situation requires you to do one
of the following:
• Remain in the sea zone and conduct combat,
• Leave the sea zone, load units if desired, and conduct
• Leave the sea zone, load units, and return to the same
sea zone to conduct combat (you can’t load units while
in a hostile sea zone), or
• Leave the sea zone and conduct no combat.
Page 22, Europe Manual 2nd edition:
Phase 4: Noncombat Move
In this phase, you can move any of your units that didn’t move in the Combat Move phase or participate in combat
during your turn.
You can’t load units from Japan with the transports in zone 6 because according to the bullet points, your transports have to do a combat move, either to escape the zone, to get out of the zone to avoid combat, to stay in the zone for combat, or to leave the zone, pick up ground units, and conduct amphibious assault in the combat movement phase, which could include back in zone 6.
The noncombat rule from page 22 says if a unit participated in combat (which is necessarily true whether you moved your transports during combat move or not, because only aircraft can move in both the combat and combat movement phases) then it can’t move
“Transports can move to friendly coastal territories
and load or offload cargo, unless they loaded, moved,
offloaded, or were involved in combat during the Combat
Move or Conduct Combat phase.”
Your transports must necessarily move during the combat movement phase, whether staying in the zone to conduct combat or to move away, so they can’t move in the noncombat phase. Moving doesn’t just mean to another zone, it means it can’t move to shore to load units either.
And in case there’s still a doubt:
“Once these sea units have moved and/or participated in
combat, they can’t move or participate in the Noncombat
Move phase of the turn.”
You can’t participate in the noncombat phase of the turn, which would certainly include loading or unloading transports. They can’t do anything after conducting combat, same as anything else except aircraft.