1. They should book the first night in SJPdP, especially if they're arriving in the evening. There will be lots of people there, and it's no fun having to find lodgings after traveling so long.
2. My wife and I are the same age as your parents. It took us 35 walking days (plus a couple of rest days) to reach SdC, but we didn't do the entire Frances in one trip. But the answer depends entirely on what kind of shape they're in. It's almost 500 miles. Even though the walking is fairly easy, how far are they comfortable walking every day? Day after day.
3. Plane is certanly the fastest, easiest and probably cheapest way from SdC to Paris. Check out EayJet, Ryanair, a well as the major airlines. If you book way in advance, you can probably get something so ridiculously cheap that they probably won't even mind forfeiting the tickets if their travel plans change.
4. Don't know the answer to your question about Lourdes. You probably need to research the options, whether it makes sense to go Santiago - Lourdes - Paris, or Santiago - Paris - Lourdes - Paris. Good place to start your planning is Rome2rio.com.