The consensus seems to be currently that Ankaur Maro cannot add to threat by adding trays, but I am still having problems wrapping my head around why this is. Please help me clear my confusion:
1. His ability simply him to add trays to any Waiqar infantry unit.
2. According to the "Adding Trays" rule, the tray must be connected so it is in the back rank. That makes it clear that if the unit has more than 1 rank, it must go in the back. No problem here.
3. If the unit has only 1 rank, that rank is both the front rank and the back rank, and that back rank is full because its got the same number of trays as the front rank (see rules 66.1 to 66.4).
Now the "Adding Trays" rule says "If the bank rank of the unit is a full rank, [...], the added tray can be added to the unit's back edge,..." (emphasis mine). Critically, it doesn't say "must" or "can only", which I would interpret that I am allowed to add it to the first rank as it will be (also) in the back rank of the unit.
What rule am I missing or misinterpreting that prohibits me for doing this? Thanks for your help!