In the case of the British 17 lb AT gun, did the breech automatically open and eject the empty shell casing after firing, or did the gunner have to manually open the breech to reload? I notice in drill position there are only two men directly behind the shield - the gunner and the loader; everyone else is either handling ammo or supervising.

Looks like a auto eject. If this right gun you are talking about. https://www.youtube.com/watch?v=KosCsRbAADg

I see them opening the breach, and the spent casing being ejected.

Definitely No!
In the footage at 2min46 you can see after firing the loader was opening manually.

Looks like I'll need another guy at the breech. Thanks.

The Gun in the Pathe clip is a Pheasant, a 17pdr on a 25pdr carriage, an expedient measure deployed first in Tunisia...

It has an auto-close breech, (as does the 6pdr & 25pdr, btw), manually open breech, insert shell, (with closed fist to preserve fingertips), & it auto closes.
Manually opening the breech auto-ejects the shell.

The carriage is irrelevant as it was the operation of the gun/breech that I was questioning - the guns themselves all operated the same.
That video was all I needed.