Does taking levocetrizine b4 boarding flight prevent nausea and head ache, and the ear pain during landing?

Unlikely to help: If you have motion sickness, something like bonine or Antivert would work better. If you have ear pain during descent, a nasal decongestant spray used prior to descent may help.

Open tubes: Eustachian tubes close up and don't want to open. Ask a pilot for the ways they get their Eustachian tubes open - they can show you easier than tell you. What you are describing is a result of the tubes staying closed. Medication will not help, only opening the tubes work.