Good questions.
>> Q1, The suffix of bpmn2 file is .bpmn2 in the jbpm-bpmn2-5.2.0.Final-test.jar. but the suffix of bpmn2 files which are created by BPMN2 visual editor is .bpmn? <<
Both .bpmn and .bpmn2 are valid file extensions and are interchangeable. The runtime can handle one or the other or both.
>> Q2, In the bpmn2 file of jbpm-bpmn2-5.2.0.Final-test.jar, the process definition tags don't have any namespace and the files couldn't opened by visual editor correctly. as below: <<
The reason for namespaces being added by the jBPM Web Designer is that it uses a different parser than the Eclipse tooling. Both generate however valid BPMN2. Designer is able to open processes created in Eclipse and vice versa. If your process for some reason does not show, the existence or non-existent of namespaces is not the issue, there is something else going on. Please check the server logs and report back..we'll be happy to try to help you resolve problems.
As far as which editor to use, that is really your decision given your requirements. Eclipse editor is targeting developers using Eclipse where as Designer targets both developers and business users, is web-based and is integrated with Guvnor which brings a ton of extra features.
Hope this helps.