I'll take the assertions of those who know him that he's in possession of his faculties and chose this course.
Since he did have a deal set in place I don't see where a daughter authorized with guardianship would get off on changing such a promise on her own.
As you note, she's been open about pressing him to strike a money-making deal. But to have done so on her own volition and without his express consent strikes me as grossly unethical.
For the time being it appears he made the final decision and consented to reveal himself without notifying Woodward and Bernstein first.
It may be expecting too much to expect honor among theives. We don't know all the facts and what the understanding was between them.
At this point, W&B would have benefited more if Felt had taken his secret to the grave. W&B would have written another book revealing his identity and harvested another ton of money in book sales and speaking fees. Felt would have received nothing except a pat on the back.
His daughter would receive a few invitations to the Today Show, Larry King, et. al., to talk about what a hero her father was. She would show some family photos and her 15 minutes of fame would be finished except for her appearance before some Leftist groups to accept awards on behalf of her father.