One book was mailed first class. It was never scanned, and since it's been almost 3 weeks, I assume it's lost. (It was only going about 50 miles away!) Once the deadline passes, what happens?

I have another book that's listed as "delivered" by the delivery confirmation, but hasn't been marked as received after more than 2 weeks. I've PM'd the person with no response. What happens in this case, once the deadline passes?

As far as I know, with DC and PBS printed postage you get your credit no matter what.  I had a book never marked received, but DC said delivered.  It turned out the member was not active, I'm not sure what PBS did about it but it left my screen and I got my instant credit when i mailed it.

As for the one not being scanned.  I think its up to USPS if they scan it or not.  I've heard that some don't scan until its delivered.  Not sure.

Be patient with the first book.  I had to wait more than 4 weeks once but it eventually got scanned and delivered.

As for the second book, not much you can do if the person is not responding.  It's possible the book was delivered to the wrong address, but it does not make sense that the person wouldn't respond to your PMs.  The requester could be out of town or had a family emergency too, ya never know.  If it goes on a couple weeks more you may want to send a site feedback explaining the situation.

At this point you've done what you can.  Don't sweat it - you got your credits and you've fulfilled your obligations!

If an account is closed or perceived inactive, the system will mark a book as received.  They could just be on vacation. You can see if their account is on hold by clicking the PM button by the transaction and then clicking on their profile or their bookshelf.  I had one recently that was a WL book I posted.  I noticed it had been scanned as being delivered 10days before and still hadn't been marked received.  The person PM'd me to say they were in India and that a neighbor had they're mail. It was marked received a few days later. She PM'd again saying the neighbor had opened it and emailed her that the book was there and in good condition. 

It still may arrive, but once the deadline passes, the system will declare the book "lost."  The requestor will get his credit back at that point.  You should PM the requestor to be sure it hasn't been received.  If he confirms that it hasn't arrived yet, you can then fill out fill out the postal form mentioned in this help doc: A book I sent is lost in the mail!

If the book arrives later, the requestor must mark it received from his transaction archive.

Oh, and if the requestor doesn't respond to your query within a week, you should go up to the Help Center and click on Contact the PBS Team.  They'll look into it.