"I want to change the design that has been used in the corners in Encore."
In Encore Shape and Encore Hoop, when two embroideries are selected, the embroidery that is first in the stitch-out order will be placed in the corners. If you want to use the other embroidery in the corners, use Stitch Order to change the order.
"Not all of my designs have been used in the Encore function."
A maximum of two designs can be used with Encore Shape and Encore Hoop. If more than two designs are selected, only the first two that were loaded will be used in the Encore. Either use Encore Circle or Line, or group some of the embroideries before using Encore.
"I have used an odd number of repeats and the center of rotation is not in the same place as the center of the circle after I right-click."
This is because the shape of an embroidery becomes a factor when there is an odd number of repeats. For an even number of repeats, the shape of an embroidery is cancelled out by the exact same shape on the opposite side of the circle. For an odd number of repeats, the circle is not 'balanced' in quite the same way, so the center point of the final embroidery may not be in the same place as the original center of the circle after you right-click.
This may also happen with an even number of repeats when the Mirror option is chosen for Position and the embroidery is not horizontally symmetrical. If you regard each pair of mirrored embroideries as a single item, an odd number of pairs may cause the center of rotation to move.
"The Mirror option does not seem to have any effect."
If your embroidery is horizontally symmetrical before you use the Encore function, the Mirror option will have no apparent effect. However, it will affect stitch order of each alternate copy. For example, clockwise stitching will become counter-clockwise, and so on.