Melusine is by far my favorite. It's interesting that you see Venus in a springy, Aphrodite-from-the-waters way; I see it as slinky, sensual, black velvet.
Have you revisited Alcyone, Idony (goddess of love & spring), Nereida, Nerissa? Or Anthea, Amalthea, Euphrosyne (with the merriment connection in your description of her, also not a stretch to Roo), Ione, Philomela?
I also like the Finnish name Virva, which of course sounds like the English word 'verve' and even 'verdant' but means 'sprite.'
I look at your description and see three elements: mystical water, sprightly nature, and shining light.
What you have to look at is which name you would use as a first? Does the first name still need to lend Roo as a nn?
First comes water:
Melusine: my favorite, because being from Luxembourg it's the tale of the capitals origin. Many fairytales surround it and it invokes the serpent-queen that was for me. It's also rather accessible while still being unique.
Next comes nature:
Willow: It's rather straight forward for nature but it obviously invokes the willow tree
Emer: Irish for swift, which is what a willow feels like for me. It's rather difficult to explain. Emer is also the beginning of Emerald, such Roo be more fiery as she matures.
Bryony: It feels rather thorny like Briar and not as swift and soft like a willow.
Aurora: dawn. Straightforward.
Aurelia: gold. golden like the sun.
Melusine Aurora Emer
I do think that if you pick a name that is pretty long it should have another available nickname in addition to roo.
Because when she's older for example if she has a name like 'Theodora' she might want to go by Thea(or Dora) rather then that or say 'Fifi' if she was called that when she was younger. (I know some people associate that name with 'poodles'! but although it's a bit babyish it's not a bad name for a child because they can have slightly babyish names.) Not that i think all names need a nickname however even if a lot of people do think that.