I think you would struggle to find any serious Constitutional scholar who would agree with your interpretation. “Except in cases of impeachment” is clearly a limit on what cases a president has the power to issue a pardon, not a retroactive “unpardoning” of cases after a president has been impeached. In fact the retroactive nullification of a pardon seems to fly in the face of a basic judicial principle that hold decisions to be final.
I think you would struggle to find any serious Constitutional scholar who would agree with your interpretation. “Except in cases of impeachment” is clearly a limit on what cases a president has the power to issue a pardon, not a retroactive “unpardoning” of cases after a president has been impeached. In fact the retroactive nullification of a pardon seems to fly in the face of a basic judicial principle that hold decisions to be final.