First of all, if your complaint is that "populism" is not even remotely the best term to use for that quadrant of Nolan Chart, welcome to the club of pretty much everyone other than Nolan. Matter of fact, Nolan Chart Wiki entry explains:
Bottom left – the antithesis of libertarianism, corresponding with those supporting low economic and personal freedom. David Nolan originally termed this philosophy populism, but many later renditions of the chart have used the label statism, authoritarianism, totalitarianism, or fascism instead. Many libertarian groups have labeled that section as communitarianism.
Why? Precisely because, while you can possibly make a case that populism (as is usually understood today, and as you outlined in your question) has some correlations with statism, it is, indeed, a bad term under usual definition, since it does not clearly imply authoritarianism despite some correllations.
Here's a modern academic definition of "populism" from Wiki:
Nonetheless, in recent years academic scholars have produced definitions of populism which enable populist identification and comparison. Daniele Albertazzi and Duncan McDonnell define populism as an ideology that "pits a virtuous and homogeneous people against a set of elites and dangerous ‘others’ who were together depicted as depriving (or attempting to deprive) the sovereign people of their rights, values, prosperity, identity, and voice".
Now, to try and see why Nolan may have used the term, and answer your sub-questions:
Is there, then, a theoretical basis for a strong link between populism and totalitarianism?
Yes, though it's hard to prove it to be "strong". Short version is that any ruler needs support from some demographics as his power base, and lack of active opposition from the majority of the populace. Even if you are a strongman dictator, your power structures need to support you, and therefore need to be rooted in demographics that supports you. For examples of what happens when you fail to arrange that, have a chat with Hosney Mubarak of Egypt (once the bread prices he artificially kept low became impossible to suppress) or Al-Assad of Syria (once his Sunni patronage networks decided that they had more to gain from deserting Alawites).
Is there, then, a real world evidence for a strong link between populism and totalitarianism?
Absolutely. The quintessential example is Hitler, of course (he was a brilliant orator with highly populist policies - public works to fix unemployment, take property from Jews and give to everyone else, "you are the superior race", revenge for Versailles, etc...)
But many, if not all, dictatorships/authoritarian were the result of revolutions or transitions that had populist policies (Italian Fascism, Bolsheviks in Russia, Roman emperors, Putin's Russia, etc)...
Are there examples of politicians using the term "populist" to describe themselves, rather than being described thus by their opponents?
This is covered on the Wiki:
Populism, unlike conservatives or socialists, populists rarely call themselves ‘populists’ and usually reject the term when it is applied to them (src: Canovan, Margaret, 1981,Populism, New York and London: Harcourt Brace Jovanovich, p.5)
Although "populist" is often used pejoratively in the media and in political debate, exceptions to this do exist, notably in the United States. In this case, it appears likely that this is due to the memories and traditions of earlier democratic movements (for example, farmers' movements, New Deal reform movements, and the civil rights movement) that were often called populist, by supporters and outsiders alike. It may also be due to linguistic confusions of populism with terms such as "popular". (src: Boyte, Populism and John Dewey)