Neutral iv Lyn

If you like her play style then don't give up on her. She might not be the optimal pick for the arena but now there are plenty of other events where she'd be viable. This also lets you harvest more hero merit which is always good and - as you note - have another option for squad assault. Neutral IV definitely isn't terrible and she's currently the only hero who can use brash assault optimally (thanks to desperation on her sword) so that's a interesting niche. If you pull another Lyn with better IVs down the road you can always merge them and keep your investment of learned skills.

To add on what Kaihaku said about merging, if you feel like you want to use her as fodder, I suggest you don't since she doesn't have anything special to inherit. Defiant attack 3 might seem good but it's basically a budget option to use over Death blow 3 and it doesn't stack with other buffs since it's a visible stat bonus. Same applies for Galeforce, it might be good on quadstriking builds but they typically want Luna to hit incredibly hard.

I use +HP/-Def Lyn (basically neutral IV's) and she's very good. with a dancer, destroys everything!!