As for importing mirrored images, just import all of your frames for one direction, select all the frames, copy them, paste them in the other direction, and flip them all in the image editor.
First of all, thanks for all the advice
I still think that the mirroring should be put directly into the RMB menu for selected frames. Having to go through the picture editor for each and every frame is really time-consuming. If I could just select the frames, RMB - Flip Frames in X, it'd be a one-step task compared to a potentially 20-30 steps task.
Also, it'd be neat if we could just copy the hotspot from the previous frame or copy from the next frame.