From the official Arlington National Cemetery website:
>>The Army National Cemeteries Program, consisting of Arlington National Cemetery in Arlington, Virginia and Soldiers’ and Airmen’s Home National Cemetery in Washington, DC, are under the jurisdiction of the Department of the Army. The Secretary of the Army consolidated authorities and created the Executive Director of the Army National Cemeteries Program to effectively and efficiently develop, operate manage and administer the program.<<
As such, it can probably be argued that those two cemeteries are, in fact, military installation under the command and control of the U.S.Army. That being the case, your only rights while on that reservation come, not from the Constitution, but from the Uniform Code of Military Justice. And, yes, you civilians who are on a military reservation are just as subject to the UCMJ as those who are in uniform. The only difference is that, if you commit a violation, you will be taken in front of a federal magistrate instead of a court martial.
I would also not be so very sure that the Guard's weapon is unloaded. As someone else pointed out, those guards are not entirely ceremonial.