Some people have had some reliability issues with their XEP's locking up - if relying on it for monitoring, I would be quite tempted to find a great watchdog example Spanky showed a while back that uses an output/relay in-line with the power to the XEP that can reboot it upon loss of comm.
Correct me if I'm wrong, but this seems huge! Using an ABN, I believe the Elk will pretty much always think it successfully communicated with the CS because the ABN simulates it - therefore limiting the Elk's ability to run backup rules... with this method you get the simpler IP option most of the time, with the ability to add a Cell backup like the Uplink Anywhere for a more reliable solution.
Or - you could probably use something like a Cradlepoint router with a 3G backup and get the same effect it would seem... making this even simpler? Spanky - if there's a momentary disruption in IP connectivity while the 3G backup connection is established by the router, would the M1 retry a couple of times?