Hi Mariah!
1. Just remember that hotels 'being booked out" on Booking.com mean solely that the rooms available for that webiste are gone, not ALL the rooms. Usualy you can try other sites (such as expedia, hotels.com, agoda) or even call the hotel yourself.
If you feel more tranquil booking in advance, do it for the first and second nights out, maybe. After that, tyou will know your walking pace better and will also have seen how crowded the camino actually is (which shouldnt be much).
2. Havent walked that variant, but just heard a talk about it last week. Sounds delightful.
3. God knows about the weather. As long as you can layer your clothes in case of cold, you should be fine.
4. I usually leave mine on the designated bag areas (if there are some) or on the floor, near the bed. I usually have a small lock to close one of the pockets in case I have something I want to make harder to steal, but any essentials (passport, money, phone) are always with me in a waistbag.
5. I walked with my husband, so wouldn´t know for solo. But it seemed very tranquil.
6. When I walked in 2017, didn´t hear about bugs in any place. Some dirty accomodation near Cesantes, but nothing horrendous.
7. Maybe train or bus back via Porto. it is a bit time consuming though(8h-10h). If you don´t mind using a bit more money, you can fly from santiago to Madrid and from there to Lisbon (around 3h if you get a good connection).
8. Porto is amazing (do a wine tasting and eat Francesinha!). I also loved Pontevedra. You don´t have to decide on stops now, just see where you feel like it
But really, enjoy the food.
9. Carry safety pins.
It is normal to be camino anxious, but once you get there, you will see how it is actually simpler than it looks. Enjoy, and bom caminho!