Is the bank's giving of interest for banking services, while ignorant of whether it is for the bank or for actual services, considered a sin? And is interest that does not multiply with increasing duration considered usury?

It is not permissible to deal with usurious banks unless there is a necessity, provided that the transaction does not involve usury. Most of what these banks take is forbidden usury, even if it is called service fees. Administrative fees or actual service fees are considered wages, provided they are specified before the work begins. It is not permissible for them to be a percentage linked to the loan; otherwise, it would be interest and not a wage. Interest is forbidden usury, whether small or large, and doubling it by extending the period increases the sin.