Speculation at this point but the clues seem to hold:
Picard didn't break Agnes' connection with the BQ quickly enough. She was assimilated, as the episode title suggests, but the transformation is not immediate. It will take time to complete. That said, Agnes is the future Borg (from the Confederation timeline?) who calls for Picard and boards the Stargazer. Hence the mask; seeing her face would have revealed too much too soon.
I agree that Jurati is the future Borg who boarded the Stargazer. They hid her face for a reason, so they could have a reveal later.
But, I don’t think the Borg Queens intentions on the Stargazer were bad. She said she wanted peace but needed power first. But we don’t know why she wants the power, her objective was assumed but not clearly stated. Assuming she just stunned all those crew members instead of killing them she might not be bad.