> GCII Article 18 [2] seems to clearly make this a requirement however
Also not an expert. But my understanding is "all possible measures" and "whenever circumstances permit" have historically been taken to not apply to submarines. Largely because in WWII, we "ordered" a B-24 to attack a German U-boat who had "broadcast her position on open radio channels to all Allied powers nearby, and was joined by several other U-boats in the vicinity" following its "sinking of a British passenger ship" [1].
Also not an expert. But my understanding is "all possible measures" and "whenever circumstances permit" have historically been taken to not apply to submarines. Largely because in WWII, we "ordered" a B-24 to attack a German U-boat who had "broadcast her position on open radio channels to all Allied powers nearby, and was joined by several other U-boats in the vicinity" following its "sinking of a British passenger ship" [1].
[1] https://en.wikipedia.org/wiki/Laconia_incident