In accordance with the "Convention for the Amelioration of the Condition of the Wounded and Sick in Armies in the Field, Geneva, 27 July 1929", and specifically Chapter VI, Articles 22 and 24, indicate that the use of the Red Cross emblem in the context of the U-156 incident was inappropriate.

While the U-156's commander's intent was clear, the use of the Red Cross Emblem on a warship is in violation of articles 22 and 24.

[www.icrc.org]

In addition, "Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923", state that

"Art. 25. In bombardments by aircraft, all necessary steps should be taken by the commander to spare, as far as possible, buildings dedicated to public worship, art, science, and charitable purposes, historic monuments, hospital ships, hospitals and other places where the sick and wounded are gathered, provided that such buildings, objectives and places are not being used at the same time for military purposes. Such monuments, objects and places must be indicated, during the day, by signs visible from the aircraft. Using such signs to indicate buildings, objects or places other than those hereinbefore specified shall be considered a perfidious act. The signs of which the above mentioned use is to be made, shall be, in the case of buildings protected under the Geneva Convention, the red cross on a white ground and, in the case of the other protected buildings, a large rectangular panel divided diagonally into two triangles, the one white and the other black."

[www.icrc.org]

The key phrases are "provided that such buildings, objectives and places are not being used at the same time for military purposes." And

“Using such signs to indicate buildings, objects or places other than those hereinbefore specified shall be considered a perfidious act.”

The U-156, continuing to be a viable warship throughout the incident can not temporarily change its status from warship to a protected ship based on the decisions of the captain. In this interpretation, the actions of the bombing aircraft were in accordance to Article 25 of “Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923". The actions of the U-156 could have been considered a perfidious act.

The intent and actions of the captain of the U-156 were laudable and should be commended. Unfortunately his misuse, no matter how innocent, of using the recognized emblem of the International Committee of the Red Cross was a violation of the applicable international laws and could have been interpreted as a perfidious act.

I do not think the actions of the captain of the U-156 could, in any way, be considered perfidious.

The actions of the bombing aircraft, while somewhat inhumane/immoral, were legal under the applicable international laws. No international laws concerning warfare would prohibit military actions against a recognized military target in favour of protecting civilians and/or the sick/wounded. The only requirement, both moral and legal, is to limit the damage to civilians/sick/wounded as much as possible.
In my opinion, the incident between the U-156 and the bombing aircraft was an unfortunate circumstance.

In the case of the U-156, its actions were moral but illegal

In the case of the bombing aircraft, its actions were immoral but legal

However, neither of the parties had the intent to commit any war crimes or violate any international laws.