Post WW II but pre-nuclear?

Interesting story about submarine design. Right after the war, Charles "Swede" Momsen (famous for developing the Momsen lung) was asked to figure out a way to increase the speed of submarines moving underwater. He threw out conventional surface ship design philosophy and looked at nature, specifically the Tuna as having the least water resistance. His prototype was, (wait for it) the Albacore. Naturally, naval leaders were skeptical until he challenged the navy to a race. Not only did the Albacore "swim circles" around older subs, it outran the fastest destroyer; something considered impossible. The rest is history as all nuclear subs use Momsen's "tuna"design.

Which one are you referring to?

https://static.uglyhedgehog.com/upload/2024/3/27/304639-lumiquest.jpg

The top left examples are not providing much of an increase in the size of the source light so the shadow edges will not be significantly softened. The light is then diffused so it covers a wider area, potentially filling in the shadows from light bounced from things outside the field of view. I should note that my speedlights (Nikon SB800 or SB910) have built in focusing and they interact with the camera to change the area illuminated as you change the focal length. Diffusers will reduce the light on the subject so the flash will have to work harder. Only a problem if you are taking shots at a rapid pace.

These diffusers separate the light source from the camera a bit more, which will reduce redeye effects. But the ceiling is a LOT further away from the camera (usually).

If there is a viable ceiling, that is in my opinion the best thing to use. The only limitation is if the ceiling is not close to white. Strong colors will give you problems with white balance. Pastel colors are probably something you can mitigate in post.

Probably not.

I went to a wedding a couple decades ago. The ceilings were 30' high so I thought bounce wouldn't work well and I made a diffuser for the speedlight.

https://static.uglyhedgehog.com/upload/2024/3/25/t1-611503-2968551437_ff2301885e_k.jpg

Just see how professional it looks!!!!!!!!

It softened the shadows a bit but did not eliminate them. Have used 15' ceiling bounce and it seems to work well.

If it works for you, don't fix it. (But feel free to experiment to see for yourself -- the paper is card stock so it is stiff enough to hold up by itself. Cost is minimal).

I'm not familiar with Canon but modern cameras do high ISO pretty well so even 30 foot ceilings might be OK. High ISO will help the flash by reducing the load and minimizing any heating with frequent shots. Go to the venue and take some test shots to try it out (or just go to an event early).