Hi Dave,
Along with the suggestions you've had, assuming this is a cluster, I would try getting the webstart from any collector, if that works you can always point the webstart at the MOM to get things going to start off. You can save a copy of the jnlp that you could pass along to others as a temporary step.
After that I would do a comparison of the /product/workstation/plugins folder on MOM and collector, particularly the last error suggests the file size could be incorrect so it would be worth comparing the output on different machines.
Without this being an exact science, if the files on the collector are bigger or if there are more of them, you could do copy/replace of the collector /workstation/plugins folder onto the MOM.
Thanks,
David