Lyde: I think this should just be disadvantageous Ko. The threat of E19 in the living variation is essentially the same as starting with E18 as a first move. Besides then in this situation [current position] E19 should also be accepted...
posetcay: I have found a way to refute current answer, looks like the answer is ko no matter what. Fixed the solution tree accordingly.
Hrewsahgs: How about adding a stone for each side to make the E1 atari legitimately sente? For the defender, somewhere around J3, and for the attacker, G5 to somewhat limit the effectiveness of the H2 clamp or the G3 cut.
posetcay: I have found a way to refute current answer, looks like the answer is ko no matter what. Fixed the solution tree accordingly.
Sadaharu: Feels a bit weird to assume P1 atari as sente in the correct answer's variation.
posetcay: I agree, but I don't know how to fix the issue. If we add stones outside, black can simply connect in sente after F18 to live, making the tsumego trivial.
I have found a way to refute current answer, looks like the answer is ko no matter what. Fixed the solution tree accordingly.