Publishers are basically just some address of the node that is used to publish any item into the stream. The publisher is used to specify the address from which the publish transaction is used.
It can be used to sort out transactions made by a particular publisher address. Suppose if some publishing cost is incurred while publishing a transaction then that amount is deducted from the publisher address.
Yes, it's possible for an item to have more than one publisher, suppose if the publishing transaction is signed by multiple parties then it would be useful if multiple parties want to asset to a piece of data simultaneously.
No, this is not what is meant by publisher as that published transaction can be confirmed by anyone and we can't specify any specific address to mine that transaction.
Hope this answers all your questions.