What are the stats of a +1 S!Corrin?

Pretty sure the first merge increases atk and something else and 120 SP.

First merge is typically HP, Attack, and 120SP. However, there are variations like; Lucina, Ryoma, Nino, Eirika, etc, who get HP and Speed (Lucina and Ryoma from personal experience). Hector also gets HP and Def rather than the typical pattern, so it seems like it's different for each hero. However, from the looks of things and judging by her stats I think the two most likely are Attack or Speed for S!Corrin.

Sorry I can't be more specific, it's too early to tell and I'm not that lucky to have gotten two of her (yet... hopefully...).

Its always the highest two stats first, going in order of looking for highest in hp, then atk, then spd, then def, then res. So if you got a Corrin that is +atk -hp, it should be atk and spd that go up one point. Otherwise, it will be hp and spd that go up, unless you are -spd, then it will be hp and atk.

I'm told it goes by highest stats, the goes into a desending repeating pattern.

If you want to be sure, go into the merging screen and click on the icon where you have
(Before) -> (After).
The (After) in that forecast will have the merge bonus applied, so you can confirm what you'll get before actually merging.