A lot of questions there Cutecumber. No expert here, but this was this years experience. I grew shallots from seed and from sets.
I got some seed from Piglet for Banana Shallots. (Thank you Piglet). I loved them, tended them, fussed ridiculously over them. Germination was poor and the wastage rate of seedlings was high. They seemed to keel over and die. It was always too hot, too cold, too wet, too dry. Real Prima Donnas. I eventually ended up with about a dozen good looking plants which are now growing nicely (but I'm not going to get that £10 Piglet is offering for the longest one)
I also planted some sets for shallots in March. No problems. They just grew on outside with no attention. They only split into 3's - but they were very big. Some have already been eaten as whole ,roast shallots, some have been made into caramelised shallot (thank you DDL, very delicious) and some are drying in the shed for ongoing use. I was pleased with the outcome. You can see the pic. That was the crop from one shallot set (is that the term ?). See the 50P for size. It only split into 4, but look at the size. Great value, I think.