in a Memories slideshow,

There currently no good way in Photos to view the photos properly in a slideshow, at least if we are having a mixture of Live Photos, videos, high resolution still shots. Whatever we do, there are limitations.

• When viewing an album as a memory, some of the Life Photos will be animated and the videos will play properly, but we cannot control the presentation time of the slides or videos. And we have to remember to turn off the sound, because it is not possible to play the memory without music. w cannot use our own music creations, we have to pick either the theme music or find Thome pleasing music on Apple Music.
• When viewing an album as a slideshow project, the Live Photos are dead, and some of the themes will not play the videos properly. But we can remove the background music and control the timing of individual slides.

What is missing is a presentation method combining the best of memories and slideshows - to be able to see the Live Photos and videos playing properly, while being able to control the timing of individual slides, and being able to remove the background music before the slideshow starts playing, or at least pick any song from our library, not just theme songs and Apple Music.

For the time being I am simply collecting the items I want to present in an album and are stepping through the items with the arrow keys, while viewing them enlarged. The Live Photos and videos will play, and as an added bonus I can see the titles, locations and dates in the title bar.

No, there is no way to edit the memory directly.

You can copy all the images into a slide show project though, and set it up manually.