As with any cruise, everyone will have their own opinion on what to do, which cruise line to use, where to go, what to see, etc.
For example, you asked whether Princess is worth the extra price compared to Carnival. Depends on who you ask as to whether the price is worth it or not. Some people are going to say it is and others will say it's not.
Princess has the second biggest presence in the Alaskan market, next to Holland America - Carnival has one ship. They go to different places, have different itineraries, and Carnival does not offer any cruise tours. So, there's no comparison on who does it better.
As for how much to spend on shore excursions, again, it depends on how much money you want to spend and how much you want to do. You can spend $75 on a tour or you could do a helicopter dogsled tour for $450. Everyone is different.
As for questions, this is the right place, but there are so many options available and so many opinions, you really do need to narrow down some of your choices so you can get more specific information.
Alaska is definitely one of those places you have to do your homework beforehand. Look at all the ports, see what they have to offer and what you can do. For example, if you decide you want to see Hubbard Glacier, that will eliminate Carnival since they don't go there. If you want to do a cruise tour, that will also eliminate Carnival. In other words, if you eliminate Carnival from the equation because of your specific requirements, then you don't need to seek any information about them.
If price is your major concern and you want as cheap as possible no matter where it goes, then put Carnival back on your list.
As for whether to do an inside, oceanview, or balcony - again, this is a personal choice. Some people will get the inside cabin and save the extra money for excursions, which is what we prefer to do. We spend all our time on the Lido Deck so we can take pictures from both sides of the ship. Others want to sit on the balcony and enjoy the scenery, but they only get to enjoy the view from one side of the ship. What one person loves, another will hate. But your agent can let you know the prices so you can decide if you want to spend the extra money.
Alaska is a wonderful place to visit, but 7-night roundtrip cruises from Seattle only get to visit the 'tail' and you really don't get to see the heartland of the State, such as Denali. If you want to see the inland, you need to do a one-way cruise along with a tour. Obviously, this costs more money, but you see alot more and get to appreciate the beauty more.
Lots of choices, but that's why I suggested talking to your agent first, provided they are a cruise specialist and have personal knowledge about Alaska because they've been there done that and can speak from a first-hand experience. Then at least they can help you narrow down those choices because there are way too many options. Plus, they can answer alot of your regular questions quickly so you can concentrate more on the specifics.