The romantic choices of many of us between the ages of 18-21 (her age during their actual affair) probably don't bear scrutiny.

I'm not scrutinizing her for any choices between 18 and 21.

This was a lifelong relationship, Hannah herself reached out and continued writing letters in his defense from the 1950s to her death.

An ex who later became a nazi (and then recanted) is probably an excellent example of how quotidian these kinds of evils can be.

Ex? No.

Recanted? They denied he had any nazi sympathy and claimed it was all a mistake

Later, in a 1969 birthday tribute essay “Martin Heidegger at Eighty,” Arendt penned what has generally been taken as an exoneration of Heidegger. In it, she “compared Heidegger to Thales,” writes Gordon, “the ancient philosopher who grew so absorbed in contemplating the heavens that he stumbled into the well at his feet.”

This was the accepted view of Heidegger until 2014 when the black notebooks came out

But major Heidegger scholars have responded in a variety of ways—including resigning a chairship of the Martin Heidegger Society—that suggest the worst. According to Daily Nous, a website about the philosophy profession, when Günter Figal resigned his position in January as chair of the Martin Heidegger Society, he said:

As chairman of a society, which is named after a person, one is in certain way a representative of that person. After reading the Schwarze Hefte [Black Notebooks], especially the antisemitic passages, I do not wish to be such a representative any longer. These statements have not only shocked me, but have turned me around to such an extent that it has become difficult to be a co-representative of this.

Hannah defends him as just so focused on high philosophy he never noticed the antisemitism

Recalls Adam Kirsch in the Times:

The seal was set on his absolution by Hannah Arendt, in a birthday address broadcast on West German radio. Heidegger’s Nazism, she explained, was an “escapade,” a mistake, which happened only because the thinker naïvely “succumbed to the temptation … to ‘intervene’ in the world of human affairs.” The moral to be drawn from the Heidegger case was that “the thinking ‘I’ is entirely different from the self of consciousness,” so that Heidegger’s thought cannot be contaminated by the actions of the mere man.

https://www.openculture.com/2015/03/martin-heideggers-black-notebooks-reveal-the-depth-of-anti-semitism.html

but I don't think it's a case of contagion.

Modern scholars seem to say otherwise

In a long, carefully documented essay, Wasserstein (who’s now at the University of Chicago), cites Arendt’s scandalous use of quotes from anti-Semitic and Nazi “authorities” on Jews in her Totalitarianism book.

Wasserstein concludes that her use of these sources was “more than a methodological error: it was symptomatic of a perverse world-view contaminated by over-exposure to the discourse of collective contempt and stigmatization that formed the object of her study”—that object being anti-Semitism. In other words, he contends, Arendt internalized the values of the anti-Semitic literature she read in her study of anti-Semitism, at least to a certain extent

https://slate.com/human-interest/2009/10/troubling-new-revelations-about-arendt-and-heidegger.html