For Q1, the Keplerian orbit in the sentence you refer to is a hypothetical, not a consequence.   *If* it loses angular momentum and *if* it remains in a Keplerian orbit *then* the orbit must have a smaller radius.  This follows from the equation.

Q2.  Note the the slide doesn't say that A slows down and B speeds up.  It says that friction *tries* to slow down A and *tries* to speed up B.  But if this process is done slowly and continuously, that doesn't happen.  As the energy and angular momentum is pulled from A, the orbit lowers in such a way that the speed increases.  At no time does A actually slow down.

Q3.  I'd prefer the term "added" or "gained", but yes.  Any angular momentum lost by A is gained by B.  The sum remains constant.

Q4.  Correct.

U1.  Why do you ask about $v_\phi$ being constant?  It's not.  As the ring contracts, it speeds up.  As it enlarges, it slows down.