Thank you for the feedback.
1. Currently RadMap does not support loading the geotiff image header to find the location boundaries. We have created an entry in our Public Issue Tracking System to implement this feature in future releases of the control. You can track implementation using the following link:
2. The UriImageProvider uses the standard WPF Image control to show an image. So, we can't control its rendering. You should be able to convert the geotiff image to other image format which supports the alpha channel (for example PNG).
the Telerik team