From the OP:
Suppose Flynn testifies that Trump actively directed his communications about the sanctions with the Russian ambassador. Or even that he knew about them. That would mean that Trump was complicit in highly improper, and possibly illegal, communications prior to his taking office. It would also mean that after he became president, he stood mute for weeks while his vice president, national security advisor, chief of staff, press secretary and communications director all made repeated public statements that he knew were untrue.
Bill Clinton was impeached for less. And that’s not the worst of it. What if it turns out that the discussions of sanctions were only part of a greater perfidy? What if it starts to look like Trump favored the Russians with a wink and a nod on sanctions to thank them for helping him win the election? Any evidence of a quid pro quo and Trump is toast.