Yes, those are the rules.

Absolutely correct.

Absolutely.  You don't even have to return the book if you don't want to.  Return of the book should have nothing to do with your credit being refunded for an unpostable.  If you are willing to send it back and the sender wants it back, then they need to give you the postage money or a second credit to cover that postage cost.

Yes, that is correct. (as already stated. :-)  From the HC section: There is a problem with a book I received!

You are not obligated to return the problem item at your expense, but if the sender gives you an extra credit or postage to do so, of course you may. 

Also, in: A requestor says there is a problem with a book I sent!

Remember that the requestor is not required to send the book back at his or her expense.   There is more information about this below.  The sender of a problem book does risk losing credit, book and postage; this is the incentive NOT to send problem books.