The version of the book I read has in the end notes that the "poisonous book" that Henry gave Dorian might have been À Rebours by Joris-Karl Huysmans. I don't really know enough to say whether that's true or not.
I didn't really notice flowers very much, but maybe I just wasn't paying attention or something.
Also, I got the feeling that Henry didn't quite take himself as seriously as Dorian took him, and might not have quite realized what effect he'd have on Dorian. And Dorian was quite unpredictable.
It definitely is... if you read A Rebours, you can see parallels straight away. In some ways, they're almost too close, but Wilde gets away with it because each novel functions in a different way. A Rebours is brilliant and if you like Dorian Gray, you would probably like that.
I've written several essays on Dorian Gray, and have done lots and lots of research.
So symbolism??? Where shall I start? Flowers are meant to be important - think what they mean:- intense, but fleeting beauty. Excatly what Dorian becomes scared of. I haven't looked into flowers specifically, but there's lots to work with in the novel!
I have to totally disagree with you over Henry: look back to the text, Henry says he wants to make a human psychological experiement out of Dorian. It is totally deliberate. Henry plays the devil here, as a tempter. So does Basil of course, who initially made Dorian vain. Dorian only becmoes unpredictable because of the way the other characters treat and mould him... either moulding him to their desires, or in a way in their own image.
If you're interested in the symbolism within the novel, take a look at diametric oppositionos, like real vs artificial. Even the characters who seems real-ish are steeped in theatricality.. like the Vanes who exist only on stage. Dorian is created... a tabula rasa before Basil and Henry got hold of him. What is real about Dorian? What is real in the novel at all? Look at the painting.. art and life reversed... a recurrent theme of Wilde's work and throughout the novel.
Sybil and Dorian work as art and life reversed too.. Dorian only loves Sybil when she is acting.. and accuses her of acting when she isn;t.
I'll stop now... this could go on forever! The novel is so much morecomplex than it appears on the surface...