Best unit to give it Swift Sparrow 2?

Hard question to answer. Much like Fury 3 or Life and Death 3, SS2 is just an incredible skill which is usable on a wide variety of units. The main difference between it and LnD is that you want to put it on units whose bulk you don't want to compromise (at the cost of it being player phase only), unlike the glass cannons who have nothing to lose from LnD. It'll be up to you to go through your roster and determine who that is, unless you give us a list.

There's also the option of keeping Katarina if you have a spare Tharja to feed her. Pretty sure she can hit through the atk bane with full bladetome buffs until you draw a better one to merge her into. I did a similar thing with Celica until I drew a +atk one recently during the cyl banner.

It really depends on what you have/who you like best and want to optimize. Swift Sparrow is great on a unit like Summer Corrin or Nino or any blade tome user. Or Celica. Or anyone who will be initiating, really. I like it on mages, personally.

It would be easier if you gave us a list of candidates and their IVs. Or even units you'd want in the future if you'd like to save her for now (I wouldn't use her if you have a better red mage...my -atk Katarina didn't do much before Nino ate her)

This is a good skill to give for those who have a good mix of offense and defense but would appreciate a power boost while keeping their bulk in tact, for example Corrin M.