Three questions on one post. Crop the video using a mask. There are tutorials out there for all three available here:
https://www.youtube.com/user/vigor2001 . Search for videos on the clone effects, masks, slow motion effects. Newer versions of PD have the interpolation technology checkbox in Speed Designer for use if checked and applied.
Optical flow is available in the Boris FX Continuum/BCC Effects List/Time/ BCC Optical Flow in this guide:
https://web.borisfx.com/helpdocs/ . I have one of their free standalone effects packs where particles can be created and used in PowerDirector. You need to contact them or check the links for more information and pricing.