My Swedbank mortgage agreement even goes as far as to require that I use my account with them as the primary salary account.

Does Swedbank require you to have your primary salary account with them to have your mortgage, or do they require you to have an account with them to receive a discount on your mortgage? B/c those are 2 different things... Thanks in advance for the clarification <3

The mortgage rate calculation is intentionally opaque (at least the bank clerks lead you to believe that) and it becomes just one of a multitude of criteria to evaluate my risk of default (and the bank having an advance estimation of any changes in my financial well-being).

This paragraph probably can be contested, if done in advance of signing the agreement, but if every other bank you approach have such a condition (to be fair, the only other bank I approached was SEB, also Scandinavian bank) spending time on arguing about trivialities while selling my soul to get an even worse rate did not seem like a worthwhile pursuit.

There is not a particular interest rate discount tied to this factor- once negotiated, it becomes a part of agreement. And negotiating this requirement beforehad would to submit another mortgage application.

I don't think any banks here allow you just to have a mortgage agreement without an account at them- they deduct the monthly payment automatically. It might be possible with other types of loans, but I don't think it can be done with mortgages.

Why would you ever sign that?

You get a lower interest rate. Banks generally can't require you to have an account there to get a mortgage, however they can give 'discounts' to customers that also have their accounts at the bank. Close your account and you are no longer a customer and lose the discount.

Banks frame it as "it's one of the factors used by our opaque black box algorithm to determine your rate". To find out the exact weight of every factor beforehand, I would have to submit N applications, processing of which is on the order of a week or so.

When I was 'shopping' for a loan (in Sweden) I got an explicit price list. As in "our base rate is X%, if both of you move your main account here we'll offer you Y% and if also you move your home and car insurance to the insurance company we own then we'll offer you Z%"

That... seems illegal.