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.