These are the official rulings:
Disembarking into an Overstacked Hex
If a disembarking unit would break the stacking limit you must put it in the nearest legal hex of your choice.
Disembarking & Movement Rolls
This is still considered movement so does require a movement roll if applicable (eg the hex you are disembarking into is across a stream).
If the roll is failed the unit remains in the transport, but counts as having moved. You also cannot disembark units into terrain types they can’t normally move into (eg you can’t infantry into a pond, or units with Thin Wheels into a marsh/across a stream.
Disembarking & Defencive Fire
Disembarking triggers defencive-fire. If a unit disembarks into an overstacked hex but is hit by DF in that hex, it remains on the transport. The usual rules and limitations to DF still apply, eg tanks can’t DF against infantry unless they have an SA that specifically allows it.