Jeff Sessions should do a lot of things but invoking the Logan Act against ANYONE, especially Obama would be suicidal for the administration. I don’t think Jeff is that stupid.
I just thought of another devious oriental strategy:
Meet with Obama.
In a day or two after he leaves China, announce a serious crackdown on North Korea.
Xi then gets to throw Rocket Man under the bus without greatly increasing the stature and power of President Trump.