The reason to use 907 is specifically because it's fast drying and easy to sand. Anything harder than the wood or primer sands more slowly and leaves a surface defect that is visible.
Just use more primer.
1st primer coat and point up with 907
Second primer coat
At least around here a landing is considered a deck in and of itself and minimum footing for a deck support post is 12x12" or 14" round.
So.. I have 14" diameter footings for a 4'x4' landing.
For the railing posts, you should be able to bolt a 6x6 between the joists as blocking. If you can bolt to 2 joists and a band even better. no need to picture frame the treads.
They can wrap the hanging post, they just can't use a cheap pvc sleeve.
I'd guess 4lbs would do it (using something over 7k meters per second det velocity).. though I haven't shot a stump in 20+ years.
They go sort of likw this.