Don't do brave bow... Just go with her original weapon and b-skill. Unless you plan to eventually whale her to +10, don't merge. Your Lyn has arguably the second best IV in the game, so congrats!
Thanks to you both for the info, but I'm oeaning towards a L&D3 Brave Bow option. If I am to have a defensive arena team with 2 CYLyns, I'm going to capitalize on being a quick af Physical Reinhardt, while my neutral IV one will provide "cover fire"