Routed units have no facing, see 3.3. Routed units are always flanked by fire, see 5.4.3. Yes, see 5.5.7. 5.5.4 says that retreating units must immediately stop on a friendly board edge. 5.5.8 and 5.5.11 give two cases where a retreating unit must continue its movement.
Continuation takes precedence.
The units would retreat off the board. Yes, but they are still disrupted. Disorganized units may never re-stack. Your presumption is correct. See assault line modifers.