I'll let everyone else debate the attachment question. Personally I would run a post from grade up with shoulder cuts and bolts.
However, if you want to do what is in your diagram, I say use a regular post base with a bolt and simpson nails.
I've built numerous roof decks similar to this and have developed my own sealing method that seals and holds.
Lots of guys burn rubber around the post and caulk it. Some guys burn rubber around the post and put small pieces of term bar on top. Good methods but they all leave the seam positive to the vertical plane and susceptible to leaks.
I use my router to cut out a 1/4" deep x 3/4" wide channel all the way around the post, then tack nail and burn the rubber on and caulk. Now the rubber (or your flashing material) is negative to the vertical plane reducing leak potential.
I haven't had any call-backs in years.

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