Job doesn't provide scheduleBuild2
Job doesn't provide getQueuePeriod [getQuietPeriod?]
ParameterizedJobMixIn provides both. SCMTriggerItem is not appropriate here.
Job doesn't provide findNearest
This should really be deprecated, and callers directed to use the more general Items.findNearest.
DependencyGraph works only for AbstractProject.
True; until this core refactoring is done, this mode would simply not be supported for workflows. See the third paragraph of my original description.
AbstractProject doesn't implements SCMTriggerItem
It is implement by Project. At any rate, as above, do not pay attention to this interface here.
Does workflow-plugin provides alternate way to define dependencies between workflow projects?
Other than the fact that a workflow can build another and provide an UpstreamCause, no, there is no DependencyGraph support. Again, for TriggerBuilder (and one mode of BuildTrigger) this does not matter.