I just seen the movie "The Way" and this is exactly something I am looking forward to doing alone.
I have been working on this route for a long time! I hope I can help out with a few answers.
1. Best airport to fly into and out of. (It all depends on cost and how you plan to get around and where you are starting from. There are trains running throughout Portugal, Lisbon is a large airport)
2. Best transportation once you land, bus or train? trains are always good, just depends on preference.
3. Are all the hostels/hotels safe to stay? (I will let others respond their thoughts on this but from all I know, read, seen, they are safe!)
4. Is the trip fairly safe to take alone? Nice thing about the Caminos - that's the point, you are really never alone
but it is safe by yourself.
5. I have hiked throughout Arizona, Nevada, Kentucky, Tennessee, Grand Canyon, etc...in all different conditions, but have never camped. I will do more reading on what to take. But any thing that could easily be overlooked?
6. How much cash do you take, and what currency? Currency is Euro, Alburques are not extremely expensive, hotels are more so, so depends on where you will stay and how you plan to eat.
7. Can you get by with mostly english spoken language, or should spanish be learned? Many understand English, Portuguese is spoken through Portugal and Spanish in Spain (obviously), but it is nice to be able to communicate in their language while in their country.
8. Again, I am a novice when it comes to this type of backpacking. I am looking for a trip possibly in March/April. Possibly sooner, but a Jan hike could be brutal weather conditions. It can be rainy and cold until around May. We are going in September of 2012 because it isn't too hot, rainy season has not kicked in, and there aren't as many pilgrims.
I hope this helps a bit!