"yasokada via Lists.Zephyrproject.Org"
Q1. I do not understand why step 3 is necessary.
I don't have this board, and I can't help you with Q1 or Q3, but for Q2:
you can always run this command to get help on options that are
available to you:
$ west flash --context -d <YOUR_BUILD_DIR>
In this case, you are looking for the --bossac-port option.