The passage you quote “Constance…” is a bit cryptic and hard to understand, but I would venture the following possible interpretation, that you might be able to validate from the context.

I would interpret témoin agente as meaning that Constance is/was both a witness and an active participant in the events she should write about. That would be in contrast to being an ordinary témoin or an ordinary agent, who are not both witness and participants, and might have special legal value. Here, possibly, Constance was a witness to the events (the complaints? the letters?), but also the recipients of the letters, maybe? I would need more context at this point.

I agree with Franck explanation: she participates to the scenes.

To day we can have a more common expression like :

‘Dans cette affaire, dont il a été témoin et agent,… ‘