If this is part of the same ticket then yes your remaining segments would be cancelled.
No the HKG-HKT-HKG reward is on a separate PNR.
If I were to go HKG-BKK instead and therefore not use the HKG-HKT leg would the HKT-HKG leg be cancelled?
All the reward seats HKT-HKG have gone or I would just cancel and rebook. Although if I were to cancel would the reward seat I have just cancelled immediately become avaiable for me to re-purchase just one way?