[three-phase using single phase inverters]

??? Please explain what has not been thought through.

Just as for the 120/240 3-wire case, in a 3-phase 208Y/120 4 wire you can have a situation in which you have unbalanced load on one phase and unbalanced generation on either or both of the other phases leading to a neutral current higher than it would be even with the worst case of unbalanced load only.

PV feeding only one phase (L1 to N) at max L1 current will produce exactly the same current in N. When you add maximum load to L2-N and L3-N only, you add the vector difference of those two currents to the neutral in phase with the PV current.

If you insure that the PV current is balanced, or close to balanced, then the current it will add to the worst case neutral current going to (unbalanced) loads only will not drive the neutral current above the rated value of the circuit.

If the unbalanced excess PV and unbalanced excess load are on the same phase, then you are just fine, since they will cancel.

Looking at the direction of the neutral current in the 120/240 case, it may help you if you think of a GTI output on the same phase as an unbalanced load.

If the two currents are equal in magnitude, no net current will flow in either L1 or N. The two neutral currents are opposite in direction.

Now if you move just the GTI to the opposite line conductor (L2), the direction of its neutral current must reverse, and be in the same direction as that of the L1 load.