is sufficient to find a fare which is eligible for the companion certificate. It is not necessary to have L space, an L-differential fare, or anything else. (A Z fare may also be available for a lower price.)

Only open question seems to be whether AM metal is eligible, since it sounds like OP saw an option with MEX-CZM on AM. In general, the Delta website is awful for trying to price and book complicated routings, and most things I see via MEX on Matrix involve very long connections with two stops from DSM, so I wouldn't be surprised if it is also a timeout issue. There are lots of options DSM-CZM in I class otherwise.