Hi, We are not going through Viking to do the pre and post cruise extensions. We've found that two days before on our own is a nice way to handle jet lag (more flexible schedule) and with both the pre and post extensions we mingle more with locals.
I'm a bit skeptical of the airline reliability too, so arriving in the departure city early gives me more peace of mind. On our last cruise (to Australia and New Zealand with Viking), a couple had travel issues and arrived a day late. That wasn't so bad as they boarded the ship just fine (it was in Sydney for two days), but their checked luggage never caught up with them. They were without luggage for two weeks, and pretty bitter about it. You never know...