There is at least one WOJ where he has stated categorically that Justin is dead, with an emphasis on DEAD. However, as established especially in Dead Beat and Ghost Story, being dead does not necessarily mean you are out of the game.
I find it very unlikely that a schemer like Justin would be caught without a backup plan. Elaine was in Justin's thrall for quite some time. Plenty of time for him to prepare her in some way.
1: Justin could be riding along or controlling Elaine, a la Corpsetaker. She might be aware, or not. He could be hiding in her subconscious and only controlling her when she sleeps. Justin could have informed the RCV then.
2: Justin is a spirit, a la Evil Bob and Kravos. He was well versed in necromancy. He could have communicated to the RCV through an Ectomancer (I doubt Mort is the only one of those).
3: Justin used necromancy to make himself into a litch. Harry said, IIRC, that Cowl's magic felt somewhat familiar, but dark and twisted. This seems to fit.