Grammar, Meaning of Following Sentence

Solution 1:

Here the main clause is your entire sentence

The corporate powers that be had decided last minute that I couldn't interview two key figures or use their information due to confidentiality issues and pre-IPO precautions.

It's clear that the subject of the sentence is The corporate powers that be and its verb is had decided.

On a separate note, the corporate powers that be uses the subjunctive (in case this is what's confusing you. )

I would further suggest you'd better slightly modify your sentence as:

The corporate powers that be had decided at the last minute that I couldn't interview two key figures or use their information due to confidentiality issues and pre-IPO precautions.