I have now tried everything I can think of - examples on website, but no improvement.
Not sure what you are referring to here. My suggestion was for you to simply replace your original Photo-JPEG/MP3 MOV slideshow file in your iWeb project with my test file (or one of your own making), republish the appropriate web page, and then see if the file was playable in your version Safari, later versions of Safari, aa well as, on IOS devices. It it worked, then you could decide how best to redo your other slideshows. I suggested iMovie as one possibility for creating H.264/AAC slideshows with transitions, but iPhoto or dedicated third-party apps are also available to do this.
I’ve reconstructed the jpeg photos in iMovie and exported them as a H.264/AAC .mov file and .m4v.
Did you test them in your iWeb project and if so, which pages did you update so we can test them on different systems and on different devices?
I also saved your Yosemite conversion to my desktop, as an 8KB, 1Childhood8005sec2SaveMov.m4v.webarchive, but unfortunately can’t open it to recreate in iMovie.
An 8 KB MOV, MP4, or M4V file would not contain the actual video data which is several MBs in size. If you "Option-Click" on the URL I posted (or "Control-Click" on on it and select the "Download Linked File" option), Safari should download video file directly to your default download storage area. As to a WEBARCHIVE file, the URL I posted is for the video clip itself. There is no code associated with this page so Safari opens the video reference using the the QT plug-in software. If you have QT 7 Pro installed, you can alternately use it to tell the plug-in player to save the movie to your hard drive. If not, you can copy the URL to the QT X Player to watch and/or save the opened video locally.
In any case, when I read your comments this morning a decided to try a few additional things. Captured frame images of your JPEG photos and extracted a copy of your MP3 audio track. Placed photos in an iPhoto album and the music in iTunes. Then used iPhoto (v9.6.1 under Yosemite) to create the following test H.264/AAC slideshow files:
Ian Test 1 Album
This is basically a 16:9 720p H.264/AAC version of your original file
Ian Test 2 Album 2
This is another 16:9 720p H.264/AAC file with black background similar to the above file but with photos "dropped" to the display area and scrolling upwards.
Ian Test 2 Album
Yet another 16:9 720p file version but using a photo collage background for slideshow in foreground.
Ian Test 2 Album (604p)
Decided to create a 604p file more closely matching your original file dimensions but still using the 16:9 aspect.
Only having extracted copies of photo copies, results were not the best—especially at the 720p output resolution. In addition, the automated iPhoto settings tend to over enlarge the smaller images while other routines would probably benefit from cropping and pre-processing of the photos before export. Still, the potential here may be of interest and, of course, you are welcome to download the files for your own tests. 720p files are comparatively large when created by iPhoto. However, when I transcoded the 604p resolution file, I used custom settings to reduce the data rate producing a video file only slightly larger than your original slideshow file.