I believe that they would have written the manuscript, probably by hand but possibly aided by a typewriter, and then given it to a typographer (probably one with experience working on mathematical material) who would have typeset it, cutting any new symbols required.

I'm not a historian but this seems like the only plausible process given the technology around at the time.

Yes, generally the mathematician would write the manuscript & others would turn it into a printable form. IIRC, some people (typographers) created moveable type with math symbols that would then be set in place by a typesetter; if that wouldn't work, a plate for a page would be created. Very expensive & laborious.

I believe that the "backward-E" and later the "upside-down-A" are that way because then you did NOT need to create a new metal symbol; you could simply reuse a capital E and A and put them in upside down. I can't find a citation for this. There's other history here: https://jeff560.tripod.com/set.html

Why would they? There would have been any number of typographers handling such work, and several already working for Cambridge and Oxford books...