I'm not 100 % sure that that's always the case, but in the above sentence about the officers being attacked, the "being" expresses the continuous aspect.
"officers being attacked" = "officers who are/were being attacked" (continuous aspect)
"the Party convicted" = "the party who is/was/will be convicted" (simple aspect)
So, while the participles aren't subject to tense, they're still subject to aspect: continuous: "being attacked", or perfect: "having been attacked".
I don't think "being" is added for clarity, really. "a report of officers attacked" isn't really ambiguous (as, if it was the officers who attacked, it would be: "a report of officers attacking", or "a report of officers who attack(ed)". And I doubt anyone would construe "a report of officers" as the subject; a report can't attack.)
Summary:
1. The party who is/was convicted
1.1. The party convicted (past participle as passive voice with ellipsis of "who is/was")
1.2. The convicted party (past participle as adjective)
2. The party who is/was being convicted
2.1. The party being convicted
2.2. no adjectival use
3. The party who had/has been convicted
3.1. The party having been convicted
3.2. no adjectival use