Never had one of those but could you not flip the black bracket shown on the right picture so it keeps the white bit away from the spokes? i think you'd need to set it up below the chainstays though. the white bit would be horizontal in that set up i reckon.
Seems to be your computer not able to handle it. Try to do some footage in low resolution and fewer fps and see if your computer can play it. what settings were you recording with?
I might be wrong but to me you are not jumping. Your hips remain at the same height before and after the hop. May be you need to concentrate in moving your body vertically more??
I'll go for second hand, for that money you can find a fairly well spec'ed bike
20 or 24 is personal preference maybe, many of us ride a pure trials bike 90% in the city... they can be equally fun to ride!
good luck