Otherwise, Congress' power to investigate, etc., et al, is unlimited except to the extent they, themselves, agree to limit it.
The guys who wrote the Logan Act had no interest in tieing their hands in the future should their friend not be in the White House.
Well there is that little thing called the Justice Department, exercised in Abscam (Murtha), and with Randy Cunningham.
If it was me, I would appoint a special prosecutor, say, Fitzpatrick, to investigate Pelosi to see if she had violated said act.