Wow, lots to think about! I especially love the gosling comparison. Yes, that's exactly why I didn't want it as part of chapter one: because none of the MCs appear in it, the Big Bad does. And unless and until he does what he does in the prologue, nothing else in the book can really happen.
My prologue is literally about 350 words long, so I briefly considered having it be the first scene of the first chapter, and then doing a really obvious scene transition. But, the gosling thing. I don't want The Big Bad to be the first character in the first chapter, I want him clearly set apart. I want you to meet Bill in the first chapter!
Also, my setup has each chapter "narrated" by a different character, 3rd person POV fly on shoulder, the same way GoT centers each chapter on a person. The difference, if there is one, is that my chapters are written more how the characters talk. Not heavy dialogue, because I think only Brian Jacques could pull that off, but like... one of my characters has zero contractions in her speech, and when it's her turn to "narrate" a chapter, the prose doesn't have any contractions in it either, because it's her thoughts.
ANYWAY. The point of saying all that, is to say that I just don't see the prologue scene working as part of a chapter, because it's Big Bad centric, so to shoehorn it into the narrative turn of one of the MCs would be really out of place and odd. It wouldn't be fair to Bill, for example, if during his turn to narrate he had to sit aside and let the Big Bad's thoughts take precedence!
And now to Bufty's questions, which I liked and will do my best to answer:
Aren't the good guys trying to find out who the bad guy is and why he is acting the way he is?
Not in the beginning. The Big Bad is using a Lesser Neutral as a cat's paw. He sends information anonymously, knowing Lesser Neutral will take the bait. Lesser Neutral contracts with the party to act on the information and Get The Thing. Neither Lesser Neutral nor the party have any real idea of Big Bad's involvement, or that The Thing is Very Bad Indeed. That's information that, for the time being, only the Big Bad has. (Everyone else is in it for money, basically.)
And if the bad guy 'sets things in motion', somebody is affected, surely? Do I see anything from the POV of whoever is affected?
Directly because of the information sent by Big Bad in the prologue, Lesser Neutral's greed is activated, so he assembles the party. The party consists of the six MCs: very different people with very different demeanors and backstories, but with a similar, monetary primary motivation to go on a quest for Lesser Neutral. (Several of them are adventurous by nature, but this is for them an added bonus, because the primary motivation is money).
The more I think about it, the more I see the plot as a sort of... Rube Goldberg machine. Big Bad is the one to drop the marble in the chute, and the next part of the book is all the gears and levers and bells and whistles reacting to that rolling marble.
Why do I need to know the who and why in the beginning if the unfolding tale (presumably) involves somebody trying to find out exactly that? Can't I find out at the same time?
I think because the party is so unaware of the real purpose of The Thing, and of the real impetus behind their quest, the reader needs to know. Otherwise it really is just a very linear "Go And Get The Thing" quest premise. By pointing out from the get-go that actually, a Very Bad Man is behind them being sent to get The Thing, (I think) the reader gets to keep that in the back of their mind, and sort of be like "no, no, put That Thing back where it came from or so help me" lol.