Some quick takes:
- most airlines have schedules up for about 12 months. My wife and I are frequent flyer club members of a couple of airlines. Where we have a higher status, we get an extra month of visibility compared to the public access web sites. Subscribe to any/all airlines you are considering now and put up with the spam, but look out for the sales and deals. Know how flexible you are on dates if you want a better deal.
- buses - schedule visibility varies. A couple of months is good, just the next month is not unusual. Prices vary, with earlier bookings typically costing significantly less. Booking the day beforehand can be three or four times the price of tickets booked earlier.
- trains - I found most schedules visible out to six months, but I didn't do a consistent check of this, and six months was more than enough for what I needed. Prices don't seem to vary approaching departure dates in Portugal. Not sure about Spain. That said, where seats need to be reserved, booking earlier gives you more choice.
- Accommodation - I book a couple of days at the start, then somewhere I can cancel or amend at the end. In between it varies from no booking and just walking to somewhere like a xunta that doesn't take booking, to having bookings for the next day or few days.
When is best? My wife and I are retired, and we can take advantage of good flight deals and fit the rest of our schedule around that. When I had work constraints, I would normally plan for:
- a rest day every 10 days, although I am happy to walk for up to a fortnight without one
- a 'sick' day
- at least one extra day in Santiago, perhaps two - there is a lot more to see there even just visiting pilgrim associated sites.
- a couple of days to a week to visit family and friends (I have a son living in England and worked there for a couple of years some years ago, and still catch up with people I know there.)
Good luck with your planning.
ps. I would heed the advice to search on more limited topics - none of what you asked hasn't been answered time and time again. While it might surprise you, you are not unique in what you have asked. You won't necessarily find what you need by asking omnibus questions, and concluding with 'and anything else?' Many of us started with questions like that, and you have been treated gently, in my view, compared to some others.