OK, it seems my Eclipse was broken by the lots of plugin installs and updates...
I installed a fresh Eclipse Mars RC3 for Java developers distribution. Then imported the project, and let m2e discovery install the required connectors. Everything seems to be working now. Except the maven junit luanch, which maybe has a bug:
java.lang.NoSuchMethodError: org.eclipse.m2e.internal.launch.MavenLaunchUtils.getBundleEntry(Lorg/osgi/framework/Bundle;Ljava/lang/String;)Ljava/lang/String;
at com.ifedorenko.m2e.mavendev.internal.launching.MavenITLaunchDelegate.getBundleEntries(MavenITLaunchDelegate.java:182)
at com.ifedorenko.m2e.mavendev.internal.launching.MavenITLaunchDelegate.getTestClasspath(MavenITLaunchDelegate.java:211)
at com.ifedorenko.m2e.mavendev.internal.launching.MavenITLaunchDelegate.getVMArguments(MavenITLaunchDelegate.java:133)
at org.eclipse.jdt.junit.launcher.JUnitLaunchConfigurationDelegate.collectExecutionArguments(JUnitLaunchConfigurationDelegate.java:280)
at org.eclipse.jdt.junit.launcher.JUnitLaunchConfigurationDelegate.launch(JUnitLaunchConfigurationDelegate.java:141)
at com.ifedorenko.m2e.mavendev.internal.launching.MavenITLaunchDelegate.launch(MavenITLaunchDelegate.java:98)
at org.eclipse.debug.internal.core.LaunchConfiguration.launch(LaunchConfiguration.java:885)
at org.eclipse.debug.internal.core.LaunchConfiguration.launch(LaunchConfiguration.java:739)
at org.eclipse.debug.internal.ui.DebugUIPlugin.buildAndLaunch(DebugUIPlugin.java:1039)
at org.eclipse.debug.internal.ui.DebugUIPlugin$8.run(DebugUIPlugin.java:1256)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)