I would add some points that helped me to solve this problem :
Having the local repository OK, on the other hand, turned out to be quite costly, as many archetypes would not get loaded, due apparently to timeouts of m2eclipse and very unstable communication speeds in my case.
In many cases only the error file could be found in the folder ex : xxx.jar.lastUpdated, instead of the jar or pom file. I had always to suppress this file to permit a new download.
Also really worthy were :
as already said, using the mvn clean install from the command line, apparently much more patient than m2eclipse, and also efficient and verbose, at least for the time being.
(and also the Update Project of the Maven menu)
downloading using the dependency:get goal
mvn org.apache.maven.plugins:maven-dependency-plugin:2.1:get -DrepoUrl=url -Dartifact=groupId:artifactId:version1
(from within the project folder) (hint given in another thread, thanks also).
also downloading and installing manually (.jar+.sha1), from in particular, "m2proxy atlassian" .
adding other repositories in the pom.xml itself (the settings.xml mirror configuration did'nt do the job, I don't know yet why). Ex : nexus/content/repositories/releases/ et nexus/content/repositories/releases/, sous repository.jboss.org, ou download.java.net/maven/2 .
To finish, in any case, a lot of time (!..) could have been et could certainly still be spared with a light tool repairing thoroughly the local repository straightaway. I could not yet find it. Actually it should even be normally a mvn command ("--repair-local-repository").