Congrats on taking your first cruise - you will quickly be hooked! I've done 2 HAL cruises to Alaska, but both were roundtrip Seattle. I can't answer all of your questions, but can answer a few. For the wifi and drinks package for HIA, it won't show up anywhere other than your booking confirmation. When you get your room key, the drink package will be loaded on to it so you can start using it right away (keep in mind the 15 drink limit per day, including non-alcoholic drinks). As for the wifi, you will be able to sign on to the wifi on whatever device you want to use and activate your free wifi package on there. You can then switch between devices if you want (i.e., from iPhone to iPad) because the wifi only works on one device per person at a time.
You should be fine decorating the door to your stateroom, as long as it is with magnets. I've seen a handful of people do that on the cruises I've been on, and some go all out with it.
Make sure to pre-book your free dining included in the HIA now, so you get the time slots you want. They tend to book up quickly, so you don't want to get stuck with something that doesn't work for your schedule. Also, keep in mind that the dressy nights (former gala nights) tend to be on sea days and are never on the first or last night, so those are good nights to book specialty restaurants to make sure you don't miss out on the dressy nights because those menus tend to be more upscale. You can start looking at previews of menus in the navigator app to get an idea of what will be on the menu during your cruise.
For Juneau, the only place I've ever eaten there is Tracy's Crab Shack, which had really good Alaskan king crab. It's extremely casual though, so might not be what you're looking for. Juneau is fairly small, so you may want to research restaurants ahead of time in case you want to make a reservation. Try and make it up to Mendenhall glacier if you can, it's really beautiful. Enjoy your trip!