Given an expectation of how hard it is to solve the problem....

Agreed

... like dolphins in all relevant respects excepts they also have hands (maybe as additional retractable limbs, to preserve swimming capabilities). So they should be about as waterbound as dolphins.

No they are not. They are much less waterbound than seals (watch the video), because they can move around on their hands and use their hands to cover themselves with seaweeds or somesuch to protect against drying / sun. I fully agree with you that such creatures are can bootstrap a civilisation especially if they have scientific knowledge.

Where I disagree is the point where an unmodified dolphin or a strictly waterbound (arbitrarily defined as cannot leave the water for more than 5 seconds) "dolphin with hands" gets anything done on the surface without having significant technology to start with (arbitrarily defined as anything humans could not build 40000 years ago). They would run into the problem that they have to build complex contraptions

Let's make the handle of the axe a much longer stick, and also attach another stick perpendicularly...

to perform simple tasks (felling a tree) without being able to build those complex contraptions without the help of even more complex contraptions (You cannot build what you described in the above quote without having wood and being able to work with it - and do that in a terrestrial environment, where you can not do anything in the first place, because you can not move.).