I think the basic problem here is that I have to prove a negative, which is, as we all know, impossible.

It's not impossible. Significant evidence of the negative will be obtained if performing a thorough investigation (which would be expected to solve the problem if it can be solved) fails to solve the problem. Applying this to flaws in particular steps, the useful goal is to show that something can't be done (that we won't find an alternative solution), not just that something won't work if done in a particular way.

For constructing a plan, I have another idea. Start with the simpler problem of developing technology as dolphins with hands. This hypothetical isolates the problem of dealing with underwater environment, from the problem of dealing with absence of hands.

Let's suppose that it's possible to solve this simpler problem. Then, I'm not sure that when we have a particular tiny operation that could be performed with hands (a step in the process of developing technology by dolphins with hands, such as smashing something with something else, or tying a knot), it's impossible to reproduce it without hands (much more laborously, slowly, using more people). Can you come up with a particular example of a very simple action that can be performed with hands (underwater, etc.), which doesn't look like it can be reduced to working without hands?