Seems we have a standoff of mutual distrust.
A) The FBI does not trust the public to read the manifesto, for three basic reasons: 1) nuts might be inspired and instructed by the manifesto to mount similar attacks; 2) release might malign and/or endanger specific individuals made out as villains in the manifesto; and 3) it would be used by some as evidence that all trannies are extremely dangerous nuts, leading to attacks on trannies and tranny-supporters (and tarnishing a Democrat constituency). [Whether or not they justify censorship, there is some validity to all these arguments.]
B) Much of the public, for good reason, does not trust the FBI to censor what the public can see and read.
That the FBI is still sitting on this set of writings, with no promise to release them after redacting problematic names, institutions, or dangerous specifics, tells us who holds power in the USA. It isn’t the People.
You articulated my mixed feelings about releasing anything authored by that nut. We know that she was a nut, what more is needed beyond that, except for perhaps deeper study into the circumstances and dynamics that made her into one. Which no doubt consist of politically damaging bad optics the FBI is hiding away on behalf of the current Regime. Then there would be passages and quotes the twisted nuts out there would seize upon as rallying cries 'to continue the fight' on behalf of their beloved martyr for 'the cause', which has potential to agitate and release even more craziness upon us by people who appear ready to do so. I don't know if there is a good way to release any of that, what consequences it would start into motion.
Calling it "The Manifesto" sounds rather confined and minimalist, going by the amount of work she generated which sounds like it would fill a library shelf. More like "the evidence", "the plans" or something.