How could Bernie Madoff run a massive Ponzi scheme right under the SEC's nose without expecting to get caught?

He didn't invite the SEC to audit him. Theranos did invite the FDA. The FDA audited them and found issues where they think Theranos has to improve and Theranos plans to improve on them. That the system working as it should.

It's not a problem for Theranos business model that they will have to add a few extra processes to raise quality. The system is working as it should.

What I know is that their business consists in a product based on an alleged technological breakthrough that they were never been able to conclusively demonstrate.

That's because you are uninformed because you believe media that tries to trash Theranos. To date Theranos got two FDA approvals.

And they are trying to sell to non-experts who aren't able to evaluate the quality of the product that they buy.

While they do plan to sell to non-experts they want the FDA to be the judge over whether they can sell the product and that also means the FDA will regulate what Theranos can write on the label of the tests.