If I'm not mistaken, the main difference between dolphins of today and the speculative species is the fact that they have tentacles. Now, when I think of tentacles, two things come to mind, the second being octopuses.
Octopuses are highly intelligent invertebrates that posses the capability to use tools. Unlike dolphins, however, they are mostly solitary animals and only ever interact with each other to fight or mate. The Pacific Striped Octopus, however, does exhibit different social behaviour, and can be seen forming flocks (herds? schools?). Unfortunately, this species is particularly cannibalistic, and due to that, (and a variety of other reasons, including the fact that they lack the intellect for anything beyond their current lives) have not formed any sort of society.
That being said, octopuses are not dolphins. The latter exhibits significantly less cannibalism, and their social interactions are more refined. One could say that dolphins are already hunter gatherers.
a member of a nomadic [species] who live chiefly by hunting, fishing,
and harvesting wild food.
Dolphins are nomads, they migrate yearly. They hunt, and have shown some quite advanced hunting techniques, and don't have any use of harvesting since they are solely carnivores.
They won't make it past this stage of sentience partially due to that fact. The next step in development of civilization is the understanding and implementation of agriculture. Without a dolphin needing to raise plants for its own food, it is highly unlikely it would ever conscientiously be able to raise food for its livestock. Even so, assuming the could get aforementioned livestock, they would have no way to contain it. Octopuses have had tentacles for millions of years, and their use of tools is primary, meaning they've never used a tool to create a secondary too, as any sort of fish-containment apparatus would require. Hunting is the only way of life that makes sense for an animal so developed for it, and with such a rich supply of available food they will never be driven to stay in one place or attempt to produce a readily available food source on their own.
Short answer: No. Not without cognitive ability to skip the step of sustenance agriculture (meaning greater cognitive ability than a human). Tentacles won't help either without the cognitive ability to make secondary, tertiary, or any tools beyond that. Dolphins simply aren't smart enough to make advances towards society, no matter what physical capabilities you give them.
Make them smarter? Sure. A sentient dolphin species with appendages could go very far.
If you don't believe dolphins are intelligent enough or have the
environmental drive to build a civilization, what could be their
motivation for developing a society and civilization?
If there was a reason proto-dolphins had to stay in one spot? Food availability. Maybe some sort of resource that causes fish to only be a viable food source in one area. After that its up to the geography of the area, and how the proto-dolphins utilize it. Here is a good resource of how a culture develops based on their geographical advantages. You can search all the episodes on youtube.
According to Dr. Jared Diamonds theories, if their environment didn't have the proper stimulus, the culture either dies off or doesn't advance. The biggest need obviously, as I’ve already said, is food.
What would be their capabilities & requirements vs humans?
They don't eat anything that isn't meat. Any physical requirements beyond that are either brutally obvious (they swim better than us) or surmountable with little effort.
Would an animal of this type thrive well under conditions of, say, Europa? Obviously, they wouldn't use blowholes to breathe when living under miles of ice.
A warm-blooded animal, or any animal really, simply cannot survive in a place that cold. Heating Europa to a survivable temperature would either require some sort of fire (generally requires oxygen. i.e. not gonna work underwater) or electrics. Unfortunately for the dolphins, unless they had labs that had weren't filled with water (entirely possible, we have vacuum labs and such) they would be inhibited by the fact that electrons travel freely in water. They probably wouldn't develop electrical technologies till much further down the tech ladder than humans did.
All in all, it's not a terrible idea, but it seems like if you're willing to change their mental capabilities to give them civilization then you're just nulling their greatest attribute. You could just as easily give intelligence to meerkats or some other animal possibly more capable of applying their other gifts.