Blessing Braves is fine, especially on a Canto unit like Haar who can make use of Red Tide twice with them.
You can Arms Scroll staves, you just have to equip them. I'm pretty sure I've done it with Micaiah.
I'd recommend Rafiel over Reyson, but it's your party, of course. The maps are so short that Rafiel's first turn of quadruple vigoring is much more helpful than the bit of extra movement Reyson provides.
I agree that blessing Wyrmslayer is a waste for the most part. Alondite lets Mia make use of Tides better but I don't think it makes a big difference elsewhere. Dragons can usually find people that don't counterattack if you equip Alondite.
Soren kind of blows by endgame but I know how it feels to leave behind a unit you've been using all along.