Comment 2 for bug 1531070

This doesn't appear to be a bug in plymouth, but in dpkg. The plymouth package in xenial (which is the one that was being installed - plymouth 0.9.2-3ubuntu8, not plymouth 0.8.8-0ubuntu17.1, despite what the autogenerated bug says) declares a Replaces: libplymouth2, which should cause its apport file to replace the one from libplymouth2 without error.

As an aside, I'm not sure what commands you ran within synaptic to get your system in this situation, but recovering from a manually-installed package's unsatisfiable dependencies should not involve installing packages from xenial onto a trusty system.

I'm not sure if there's anything more here that we can do to facilitate recovery now that your system is in this situation.