Hi Tom, for the first issue, the extra slots shouldn't be there cus we removed the middle beam later on as we think it was unnecessary. We forgot to remove the slot however, so there's such a slot. You can choose to make a beam in the middle using the 3mm sheet.
For the knees, normally they should be labeled in a ascending order from bow to stern, this one was special however, so that there was a labeling error in the instruction. Yes the forward one should be M15 and aft one should be M14.
For the inside planking, the order should be:
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sorry for the late reply, was unwell last week so i didn't browse SOS much, instructions will be updated, i hope all these helps.