Too merge or save for SI

Keep. As a F2P, I only merge if they have great IVs, or a lot of SI in them. And a +2 isn't very good, versus two Swift Sparrows.

Second that, save for SI.

Here is yet another reason to save as fodder;
Brave Lyn is cancer.
Honestly, I refuse to use her unless I absolutely have to, and only in Single Player content. For me, using a unit like her takes all the fun out of the game due to the fact that everyone is using her.
It's up to you, but Swift Sparrow is too good of a skill not to fodder. Many units can make great use out of it.

I'd say merge. B!lyn comes around often enough that you'll have a chance to +10 her if you're active patient. SS is nice for SI, but isn't a part of a lot of optimal builds on elite heroes, or if it is, it's replacing something like death blow (Siegbert).

For F2P, it is especially important to save 5* exclusive skills for SI. You should be merging units readily available in the 3-4* pool, who could use said skills to great effect.