I have tentatively given into agent (and editorial) suggestion. After much experimentation, I have finally settled upon il - ilu - ilish - ilishtum, which conjugates in the same way as he - him - his -himself. You have no idea how much my teeth are grinding, but if it helps sell the book, so be it. Right now, I'm gripping my head, hoping with all my might that I don't lose half my readership in the first chapter.
Thank you, John Huehnergard. Without your in-depth and easily understood knowledge of a murderously complex dead language (which I happened to usurp for my nefarious purposes), I'd be hanging by my fingertips right now. As it stands, I've got at least one finger wedged into a fissure at the edge of the proverbial cliff.
Tonight definitely calls for a drink.
ETA: make that iliš. It's pronounced the same way as ilish, but is far less visually distracting IMO. Ah, special characters. How did I know it would eventually come to this?
I still want my beer.
ETA2: Aaaad now ilu is ilum. Will the madness never end???