How to assemble jpegs into a movie and set image duration

It never worked this way. The way it has always worked and still works - when not using an image sequence - is to add one, change its Duration in Properties, click Set Default next to Duration, then add all the others to the playlist.

It is editable when the selected image or clip is not from the timeline.

That is another way that is better for large sets of images.

Incorrect. Pay attention to what is selected. If you only have a playlist (timeline is empty) and switch the player to Project tab, then it treats the playlist like a media player playlist - play everything sequentially. In that context, there really is no selection and Properties is blank.

I realize you stumbled along and got something working by now, but in case others are reading this especially after searching they could use accurate info.