Thank you so much for your insight! I know first hand of the difficulties - I have already frogged this sweater once :/
Considering what you wrote, I'm thinking of unravelling a couple of centimetres of the sleeves, based only on a wild guess. If after the wash I want back the lost length, I'll only have to unravel the i-cord. Or even skip the i-cord, wash it holding the sleeves on a lifeline and finish it afterwards. This way, I wouldn't need to unravel anything after wash. Would that work?
(Making and blocking a swatch would definitely be the adult thing to do, but I'm really not keen on the prospect of looking for more yarn...)
Thank you for the kind words!