I’m implicitly assuming Chinese-like (area scoring) rules (with strict superko) when mentioning a number like 7. However, we don’t know the exact number for certain.
If the correct komi for area rules was known to be definitely 7, then that would be very strong support that the correct komi for Japanese rules is either 6 or 7, but we still cannot say for certain that it is not something else. We do not know if any of peculiarities of the Japanese rules might create a vastly different outcome. Maybe perfect play involves building a seki with an unbalanced number of eyes that don’t score points under Japanese rules. Maybe a bent-four would emerge and be considered dead despite unremovable ko threats (like from a seki) elsewhere on the board. Maybe perfect play under Japanese rules is ill-defined since the game must end in a triple ko that neither player can abandon without losing.
The average between 6 and 7 is 6.5, which makes for a reasonable argument as to why we can use a komi of 6.5 for Japanese rules, given our uncertainty of the situation. However, the correct komi is certainly not 6.5, by definition, since it cannot lead to a draw (by jigo) under perfect play. If an oracle told us that 7 was correct for Chinese rules, and no “funny business” (see above) should happen for perfect play under Japanese rules, then we could only say that the correct value must be either 6 or 7.
I don’t understand this step of your coupon argument. Could you clarify? Do you just mean to say that the largest coupon must be equal to or one less than that for Chinese rules (because of dame filling)? Is this argument just functionally equivalent to the earlier one?
Under Chinese rules, it is possible (although uncommon) for the score difference to be an even integer. This would occur if there is a seki creating an odd number of unfillable dame points. They skipped 6.5 since that was unlikely to make a difference versus 5.5, but in principle it could.