We can increase ION's rate of successful bundle delivery in at least two ways:
When a bundle is reforwarded due to expiration of its xmitOverdue timer (indicating the failure of a routing decision that was expected to be successful; analogous to reception of a Custody Refused signal), in computing a revised route we should be open to the possibility that the best plan might be to forward the bundle back to the node from which we originally received it. That is, in this case the sending node should be omitted from the Excluded Nodes list.
Continuing in this vein: the other mechanism we've got for backtracking in the network is Custody Refused signals, but those signals are issued only for bundles for which custody transfer is requested. Since custody transfer is not mandatory but we always need the ability to send a bundle back through the network to a node that can compute an alternative route for it (the "snub" system), we should send Custody Refused signals whenever forwarding a bundle is impossible, even if no custody transfer was requested.