Which B Lyn to merge?

In my opinion, HP is her second best bane next to defense. She will be able to tank mages slightly better with the extra res despite the HP drop. I would merge into the new one.

You sure you don't have anyone who could use swift sparrow?

I got a -HP + def Brave Lyn once. IVs are terrible, but it's always great to have an extra Brave Lyn. Merging only raises 2 stats by 1 point.

+RES is better than neutral. She can't deal with high defense dragons but more often than not she's going to be hit by mages and the extra res helps a lot and boosts iceberg damage.

That said I would wait for a +Atk or +Spd unless you're planning to +10 someday.