You have 2 separate issues here: one is that you want to import slideshows that are in m3u format into Media Center and have them show up right away in Slideshows in the Picture library, and the other is a feature request to play a random slideshow.
For the import issue, I think you may be running into an issue with how we tell if a playlist is "music only", "picture only", or mixed. Since WMP has no concept of a concurrent playlist with music and pictures, the playlist itself doesn't contain metadata to tell us if that's the case. So, we have to crack open the playlist and examine items within it to tell. Since that can be an expensive test, especially over network connections, we have a compromising optimization to improve gallery load time. Instead of opening every item in the playlist, we only open the first and last items. If the first and last items are both music, we assume it's a music-only playlist. If the first and last items are both pictures, we assume it's a picture-only playlist. If they are both, it's mixed.
The reason we do this is because we don't really support importing playlists from other formats directly--we only support it via WMP. When Media Center saves a mixed playlist, it saves all of the pictures in a block, followed by all the music in a block. This is because pictures are cheaper to identify in the playlist than music is, so scanning forward to find the first music item is faster than if we had to scan to find the first picture (IIRC).
The reason you saw the behavior you saw is because once we actually open a playlist for playback, we read the whole thing instead of the higher-performing method for media-type detection. If the playlist turns out to be mixed, we overwrite the original playlist with the ordering I described above, which causes future visits to the galleries to show the correct info.
So, to get the behavior you want, save your playlist in the same ordering scheme that Media Center uses. Group all of your pictures together at the front of the playlist, and all of the music at the end. The pictures will play in the order presented, as will the music, in parallel with each other.