That is the growth which can occur unimpeded for up to 50 years.

I'm not sure those two quotes are talking about the same thing. I notice that different numbers are quoted in different statements in the document; for example, on p. 12 there is this statement:

This reinforces the observation in the 2003 MIT study that “We believe that the world-wide supply of uranium ore is sufficient to fuel the deployment of 1000 reactors over the next half century.”

Which is different from the "80 years for 800 reactors" statement.

But more importantly, you're ignoring additional information that's given in the same sentence we've been quoting and the one immediately following it. You expanded the quote to give the first half of the sentence, but you conveniently ignored the last part of the sentence, which I'll repeat: especially since resources costing several hundred dollars per kilogram (not estimated in the Red Book) would also be economically usable. I.e., the "80 years for 800 reactors" figure (and by implication the "1000 reactors over the next half century" figure) does not include all estimated resources. The very next sentence is:

Using a probabilistic resources versus cost model to extend Red Book data, we estimate an order of magnitude larger resources at a tolerable doubling of prices.

I.e., at twice the current uranium price (which, as I noted before, would mean only a small increase in the price of nuclear electricity to the end user, unlike fossil fuels where fuel cost is a major factor in end user price), we have 10 times as much available, meaning 80 years for 8000 reactors, or 10,000 reactors over the next half century. That's a big difference. And since uranium prices will certainly go up if it becomes scarcer, there will be natural economic forces driving people to tap the 10 times as much uranium that's available at higher prices. So you are conveniently failing to acknowledge information that's in the document you linked to. (And that's still leaving out breeders, which as I noted before, were left out of the MIT study for economic reasons, not technical reasons, and as uranium prices go up the economics change.)

those 21.3 million acres

Or about 23 percent of the area currently under cultivation in the U.S. (922 million acres according to Wikipedia). But we're talking about 25 to 50 years, not right now. There's no reason why algae acres have to displace current farm acres on that time scale. That 21.3 million acres is about 9 percent of the land area of the U.S, which is significant but doable. Or, if we wanted to get creative, we could put the algae farms on platforms offshore and out of the way (and with easy access to fresh water using desalinization rigs--or even using salt-water tolerant algae).

Good luck with that.

In the next 25 to 50 years, the items you list are easily doable. That's not to say they'll actually be done; but once again, the main obstacles I see are political, not technical. I notice you haven't commented on that at all.

Can you demonstrate a single case in which human ingenuity has created a new entropic gradient to exploit, rather than found an existing one?

Of course not; that would violate the second law of thermodynamics.

What existing gradients can you point to that we can tap?

Here on Earth, for the near future, you've covered all the significant ones I'm aware of (assuming solar includes solar thermal as well as photovoltaic; I haven't seen a lot lately on solar thermal, but it seems like an obvious alternative worth pursuing).