1) Stick with 120-400mm if you only have one camera body for both shows. Just be aware for the main show that for the morning and early afternoon you are shooting straight into the sun. The light (if sunny) is much kinder for the evening show.
2) Spottersday gives you the chance to see the KB based aircraft on the ground and the Saturday rehearsals depart/arrive. The Belgian Air Force aircraft sometimes do display too, but with Lens and Scampton sharing the BAF aircraft this may be somewhat more difficult this year.
3) KB is big enough for everyone! You don't need to follow the sheep are all cram in to one tiny space!
5) Expect early morning fog, rain and sunshine.....