"In retreating or advancing, the units involved must
move to hexes that are not occupied by enemy units and to
which they could move in the course of tactical movement."
Does this also include across Rivers?
Short answer, Yes.
This question was partly answered by the reply to your query in https://boardgamegeek.com/thread/1628172/movement
If a unit could cross the river if it were doing regular Tactical Movement, then it could cross if it were retreating.
So, to spell it out, a full-strength division, HQ or non-motorized unit of any size could retreat or advance across a river.
A motorized unit stacked with a full-strength division or HQ could do so also, but the division or HQ need not be advancing as well, as amplified in the previous reply:
A motorized unit that starts its activation stacked with a full strength division or HQ unit next to a river spends all its MF to cross the river, but it does not have to cross the river with the stack (nor does the stack have to cross the river).
The thinking here is that a full strength division or corps/army HQ unit will have bridging resources with it sufficient to let a motorized unit cross.