When I get a new chain, clean with paint brush cleaner. Put it in ultrasonic jewlery cleaner (cost £10 at charity shop), also cheap on Amazon. Put in melted wax for 20 mins, give it a good swoosh around till no more bubbles.
For reapplication, just put in wax, no need to clean
Re-use of quick link. I carry a spare on rides. I reuse the shimano quick link for the life of the chain. Fit a new one for races… only failure I have had was a new quick link on first ride