Severus! Yes, he would be quite fond of Astoria and would welcome her into the shop. He'd be rather liberal with the pricing when it came to whatever she needed unless it was a super rare ingredient of course. She's actually one of the few people he wouldn't mind being called "Professor" by, since he's moved on. Though it rankles him when people call him "Mr. Snape." Reminds him of his father.
Arthur! Likely knows very little about Astoria's existence, unless Ginny's mentioned her. But Ginny would likely talk to Molly about it more, I think. Also, being a blood traitor and all, I doubt the Greengrasses want anything to do with the Weasleys. And he's okay with that!
Draco! .... What can I say? The boy is absolutely smitten. Has been really since they met. Thinks Astoria is just adorable. Wouldn't mind getting to know her better. It's not love at first site, but a crush is definitely developing, and he's trying very hard to hide it.