Are you confident with this job? To me it sounds like you have many questions regarding shooting a wedding,,,
Shooting a wedding with little depth of focus, wide open, takes a lot of skill/practice, not recommended for a first wedding.
Under no circumstance would I use ND filters or polarizers for weddings, I see no reason at all.
Yes flash outdoors is very good for fill flash at weddings, and yes with diffuser, setting up SB910 correctly is mandatory for fill flash to work well,,, FP 1/250
I always used big flash SB 800 - 910 with f/2.8 zooms; 35-70 AF-D, 28-70 AF-S, 24-70 AF-S and the big Nikon Bodies or with grip; F4E, F5, D1, D1X, D2X, D3, D850 since it's a heavy setup to carry for hours,,,
Always also a complete back up set of; Camera, lens and flash, just in case,,,
Film/memory cards and batteries in the pockets, not in a bag somewhere,,,
Also often Elinchrom battery pack, Ranger RS with a big softboks to overpower the sun with soft light, for the images of the couple.
Then scouting out the church, talking to the priest for permission, what's allowed, what can you get away with, overall location knowledge is helpful for camera angles.
Then knowing who is who,,, because some pictures of the key persons are mandatory,,,
Being alert for all the key moments, arrival, brides maids! ring, kiss, flowers, exit church, rice, parents, kids, official as well as unofficial,,,
Good luck!
PS definetly NEF raw files, they are so much more flexible to edit and quick to bulk convert, and a good opportunity to save images with issues re light etc