Re the rudder reversal problem* inherent in the (small) fin design I believed it was after the the loss of a number of leading boffins in the RRE (Radar Research Establishment) Halifax from Defford that spun in from height with the same control problems that HP were ordered to sort it pronto. The fix itself was relatively straightforward - larger fin and a change in the position of the rudder hinge.
However, it was still aerodynamically possible to get rudder reversal in certain conditions on any twin fin design; it was still warned about on the Shackleton, although thankfully never demonstrated!
* Rudder reversal: If applying a large rudder input, for instance in the event of an engine failure, with a small fin and the hinge line more 'central', it was possible the airflow could 'catch' the leading edge of the rudder and force it hard over (ie full deflection) and the airflow over the trailing edge plus pilot input was insufficient force to recover it - hence the spiral in.