Because of this thread I spent last evening associating flowers with all characters of my comic, who have role big enough to have their own bubble color, but I'll spare you all the whole list and just focus on the MCs. xD
It's more based on flower meanings or "this is a X character" rather than what flower would they want to get.
Himuri would get forget-me-not. It's blue like his hair and small just like him. xD
As for candy, he'll enjoy coffee flavored stuff most, but he's not picky, so he would be happy if given anything sweet.
Sakari would probably get some white flower. White lilac maybe?
I imagine him enjoying green tea flavored candy.
For Kiyomi I find purple hiacinth fitting mostly because of it's meaning (No spoilers. xD)
And just give her any kind of chocolate and everyone's happy. xD
BONUS!
I have no idea, why would anyone give a villain Valentine gifts, but Iwao was first character I instantly associated with flower. A red spider lilly, which is cutely nicknamed "hell flower" in Japanese. xD
As for candy, probably dark chocolate. But like, really dark.