agreed. TobiahMalachi and RosalieSeraphina. Rosette doesn't mesh as well... and Tobias seems a bit out of place considering your -iah theme. Though, I like both boy options.
I think so... but other variations might work, like Rosine, Rosalind, Rosanna, or even Roselle. My favorite is Rosalba, but that won't match your style. I think Rosalie is really your best match and it is a lovely name.