Does it mean that transactions below the priority cannot be propagated at all or that these transactions cannot be propagated without a fee?
this question does not make sense.
there are rules in the network that determine if a tx can be propagated.
please read the documentation link I provided.
my understanding was that the user determines the fee and that it can differ from the suggested fee.
that is correct
If this truly is the case, then how does Electrum determine the appropriate fee,
the fee depends on the length.
it is the fee per kilobyte that you set in your preferences multiplied by the number of kilobytes of the transaction.
why does it not show it and why is it not just a suggestion?
what makes you think it is not the case?