I'll answer #1. IMHO a round trip out of Vancouver is definitely not the way to go if you want to spend any time on land in Alaska in Anchorage or Denali either post or pre cruise. Other cruise lines have one week cruises that start or end in Vancouver so you do the land portion after or before, in those cases.
We've done southbound Princess twice with land tours precruise. If you've never seen Alaska before, I wouldn't recommend skipping Denali Park, Anchorage, etc. Actually, I wouldn't recommend skipping Glacier Bay, either. And
DCL won't have access to Glacier Bay on the cruise.
For those of us who have been to Alaska before and will go again, this one week out of Vancouver could be a "fix" just until the next real Alaskan trip. I'd consider it, but would likely combine with some days at
Disneyland in California before or after since I'd be on the west coast anyway.