Thanks to Brendan. I enjoyed it a lot but twigged the concept, including the long concatenated solution, quickly. So it was a pretty quick solve over dinner.
Only quibble is that the uber-thematic pair wouldn't have the same # of letters, which led to a slight awkwardness.
FOI 5, LOI 7. 2 probably took the most time (for no good reason), followed by 9, whose solution was clear but whose parsing came slowly.
Also didn't think about COD so much, because the concept was the most outstanding feature IMO. 4 and 2 brought biggest smiles, so I'll call them co-COD.