Hi Newishcruise,
Usually for first time Alaska Cruiser, it is recommeded you do a round trip. For those who choose to sail up to Whittier either have plans to continue a land segment or spend some time in anchorage and fly out.
Can't comment on Emerald as I have only been on Royal and Diamond. You may want to watch some youtube videos to have a better idea.
With regards to booking through Princess website or Travel Agent, I think it depends on the country you are in, how established are your cruise TAs. Where I am from (Singapore) TAs on cruises are limited and I always book direct for various cruiselines.
Hope this bump up your question and there will be more replies.