I don´t feel it as a natural sentence. Even before the coronavirus hit, trouble was brewing in the land of unicorns, as tech startups worth more than $1bn are called. I would change it like... Even before the covid-19 pandemic, the problem around the Unicorns land was there, so these well-known startups worth $1bn already. - I feel this is more natural but even a native speaker may say this another way or fixing and moving some words. I hope this may help you.

It's fine, although you're right it reads a little awkwardly. It might be a bit clearer if they used an em dash in place off the comma to emphasise the final clause and make it more readable: "Even before the coronavirus hit, trouble was brewing in the land of unicorns - as tech startups worth more than $1bn are called"

The sentence is fine.

it's perfectly okay.

Hmmm... tricky.... I would replace as with “what” or "which" You could put quotation marks around “land of unicorns” to improve readability...