Taking all the above into consideration, I think they must have repaired the flaw in the same way as they sometimes filled dates and recut them, (e.g. my 1675/3/2 halfpenny, where the first modification was to fill the 2 and recut the 3, whilst the second simply added a sideways spur to the right side of the 3 at about half height). The flaw doesn't reach the edge of the coin, so the die is essentialy intact on the 1677. If it had split to the edge it may well have been terminal. The width of the flaw is no greater than a letter upright, so it is quite possible to effect a repair in this way.
Try as I might, the flaw shows no hints of being repaired on the 78. I was hoping for a change in surface, visible when the contrast is adjusted. The full set of harp strings on the 78 in comparison to the 77 and the crown cushion suggest was extensively recut, particularly given the strength of the two arcs on all the shields compared to the 77, which seem a bit weak on the 77.
As for the G over O, the same die albeit repaired covers the problem of accurately reproducing a compound letter. I would go for O rather than D based on the images.
No you haven't wasted your time, you have answered a question.