2 Romilly's surprised me too but it does have some currency over here. There's a news correspondent called Romilly Weeks, and while she's not a big celebrity, and I don't think parents would be honouring her, I think hearing her name said on the news all the time means it's in the public consciousness, and it takes a bit of the strangeness away.
I don't think
Rowan will have too much trouble since it is the way the tree is pronounced, and
Rowan on a girl is less common in the first place, so I feel people would be more likely to accept it as being from the tree rather than the same as the male
Rowan from the Gaelic name, if that makes sense.