You have read your own sources, right?

The first one, from marxists.org is an article about the Germans and why they let the Nazis seize power. The The author attests that

And the German people, intoxicated by demagoguery, driven by the scourges of terror, with barbarously perverted instincts, wandered towards death.

And uses The Magic Mountain and Heinrich Mann’s novel The Loyal Subject as examples for the then current condition of Germany. At no point does it mention anything anti-fascist about either book by the way.
It tells us that Mann identified mental conditions, the dissatisfaction, moral disgust, inability to understand why.
These are themes the reviewer identifies in the book.

That the people thus affected are ripe to accept fascism, later when it eventually comes about, is the reviewers conclusion.
It is not something expressed in the book.

Further down, other works are discussed and they are specifically called “anti-fascist” unlike The Magic Mountain.
About Farewell by Johannes Becher it says:

Although the action of this novel, like the action of The Magic Mountain, is played out before the first world imperialist war, it is inspired by the modern anti-fascist struggle.

It draws the parallel of both books being set before WWI and the distinction that only one of them is is inspired by the modern anti-fascist struggle. It is not Mann’s.

Your second quote mentions The Magic Mountain in the first line and is then entirely about The Cabinet of Dr. Caligari.

And the third one

he became such a forceful foe of Nazism

Yes, we’ve been over this. He didn’t like them from the beginning. Certainly didn’t improve his opinion when they set up their fascist regime, years after 1924.
All the same arguments about the passage of time and when things happen, that I’ve made before.