> the "application.xml" of the xxxx-project is still incomplete (no modules, see below)
You have to open the pom.xml file of xxxx-ear project and change the generateApplicationXml element from false to true
That won't be necessary when the fix for https://jira.jboss.org/browse/JBIDE-4809 is included in the build.
> I still can't build the .ear - project ("could not find artifact org.mygroup:xxxx:war:0.0.1-SNAPSHOT", as before)
You can build the ear project by rigth clicking xxxx-parent and calling Run As>Maven clean, Run As>Maven package.
m2eclipse 0.12.x doesn't support workspace resolution for a flat layout project.
If you still have issues, please attach your xxxx* projects.