1. Normally when you renew, you can get the old passport book back, either automatically or at your request, although it's often sent to you separately from the new passport. It's punched to signify that it's no longer valid. I don't know how passport cards are handled in this respect.

2. To me, it would seem to be much less hassle to renew both at the same time. Again, I don't know the rules on this as I've never tried to renew them separately. I think the fees are less when everything is done at once, especially if you need expedited service.

3. You can renew the passport book pretty much whenever it makes sense for you. Some people need new passports extremely early because they run out of space for visas and entrance/exit stamps very quickly. USA will no longer insert additional pages into an existing passport, but when you apply or renew, you can request one with more pages. If you travel a lot, you might need to strategize when you can best be without a passport for a while, although expedited service or using a passport agency (not a passport service, as that means paying more for slower service and more hassles) helps here. It also makes sense to try to renew your passport during "low season" for passport services as it could be extremely inconvenient to submit your renewal by mail and then learn that it will be several months before you will receive the new passport. The main reason not to renew too early is the fees.

You can renew rather than applying for a brand new passport if the old one expired not too many years ago. However, most of us would never want to be without a valid passport for very long.

4. No idea. Perhaps travel stores sell them? I'm not sure whether I received one with my passport card or not, but I'm not using one now, although I do have and use the special envelope for my GE card.