Excursion info: The team can help you if the, personally have already been on the excursion you are asking about or they can book it for you if you are in your booking window. Otherwise they know what is online for you to see on the
DCL site.
Decorating the cabin: This is a purchased package that is available for various occasions. This is not a complementary service, so they cannot help you.
Caffeine free diet coke: If they have it on board they will put it in your cabin. Just ask your concierge when you board. I do know that a friend requested some non-Kellogg's cereals and they were able to get some for his suite.
The shoreside concierge team can help you with many things, but they really have little control over shipboard issues. Some items, such as a rollaway bed or lounge chairs for the verandah, have to be approved by the captain and are not within their powers.
On the other hand, if you want to purchase items for your cabin, such as the decorations, they will gladly arrange it for you. I purchased some gorgeous roses for my wife through them for our Alaska cruise.
Eric, Sue, Lynn and Kristin (as well as Jamira, who is now elsewhere at DCL, and Autumn, who is now on the Dream) have provided excellent service. I am sorry you had difficulty, but non of your issues appear to be within their authority.