1) Lets say, I want to choose my own tracking plane based on some points in point cloud, would it be possible?
2) Or, lets say, I have initialized the Instant Tracker with a horizontal plane and I want to update the plane orientation, how do I proceed?
3) Or, any other way to manually set the origin or a tracker?
1) I'm not sure if I understand this case correctly but as a point cloud is only available once tracking was started and a running tracker can not be updated with a new initialization plane, I would say this will not work.
2) You would need to switch back to initiation state, let the user initialize again and then switch to tracking again. Please note that you can either set 'horizontal', 'vertical' or define a custom angle for the initialization plane.
3) Manually defining the origin of the tracker would only be possible with the Native API SDK. Here it would also not be a SDK feature but you can implement it yourself as you have all the required access/control to the tracking matrices and rendering.