Except in really critical applications like payments, etc., as a user I'd rather have the convenience of being warned early about already having an account that the little extra security.
Is saving that extra amount of work in a rare case a concern in practice? Sure, it would annoy me a bit if it happened to me, but I'd be more like "man, I'm stupid" instead of "man, this site is stupid, I'll never go there again."
Only a mild annoyance, but still more annoying than it would be to have someone access my account on such websites (which would be basically negligible, as I don't store payment information there, and you have to multiply it by the also negligible probability that the information leak from the login form is key in letting the bad dudes access my account).
If you demand lots of information that should be clear right away.
People shouldn't be making long forms part of account creation unless absolutely necessary; in which case it should already be obvious to the user that it is.
If it's somehow necessary but not obvious, you can put up a friendly warning. Maybe something like, "Step 1/12" or "expected registration time 15min."
/s, but only slightly.
You're right, of course. The answer is the one nobody wants to hear: you can tell users what you'll want from them, then ask to create an account, and then ask them to do the things you outlined before registration.
As for multi-page forms, they make sense if you target non-JavaScript use, but if your site is already an SPA, you might as well present the form in full, and conditionally disable parts that don't apply based on earlier inputs. This is the way to improve over the paper UX.
So in the email you can include a link to optional account (username/password) creation.
And if you aren't sending an email, then the user has clearly decided not to create an account.