Hi Chris, my penny's worth
1. Although lots of keepers use Virkon S in a pond I would be reluctant to use it myself.
2. The PP dip timing sounds a bit long for my liking, I have been told that you are not gaining anything by keeping it longer than 3 min in a PP dip. I would remove the Koi the minute she starts to look uncomfortable or start to stress and not a second longer. I have over done this before and it took some time to get the Koi back to normal.
I know Pieter reccomend a very strong salt dip when transferring Koi to a new pond, but I have not done this before.