Brilliant analysis GreySun! Everything is spot-on. I'd like to add some of my findings as well. Hopefully I'm not reading into it too much.

In his left hand (our right), Alucard holds out a rose. One half is white and the other is bloodstained with a bat emerging from its petals. It is symbolic of Alucard's dual nature, the white representing the beauty and purity of his human mother, and the red representing the sins of his father. Though he feels drawn towards his mother's kindly human nature, part of him will always be vampiric and will try to tempt him into obeying his father. The bat makes up a smaller part of the flower than the white, pure petals do, and the bat is positioned closest to Dracula, and is flying away from Alucard. This means that Alucard has learned to control his nature and knows what is good despite growing up under two influences.
Additional observations on the succubus: I see the thorned flowers on her head as being a mockery of the sacrifices of both Christ and Lisa. Both figures were fixed to a cross (however differently), were killed though they did nothing wrong, and willingly gave themselves up. Since the succubus imitates Lisa, the thorny flowers mock Lisa's death; the flowers and thorns combine sexuality with torture as in the analysis of the Lament of Innocence succubus. You could also take the Christian symbolism of the crown of thorns and combine it with the sexual appeal of the flowers to create an image of "irresistible torment," if you will. Also, given the crown of thorns' symbolism of a torture instrument, it could refer to her mentally torturing Alucard in his nightmare. The fact that the flowers and thorns are gray and cold, almost steely in their appearance, refer to her coldness and apathy towards her victims as she tortures them mentally and physically.
I didn't want to read into it too much, but I think I already have! I am just so fascinated with hidden symbols that it's easy for me to go overboard.

I did the same thing for a school project, with Order of Ecclesia as a diatribe against papal hierarchy, which I think I definitely DID read into a little too much, but was still pretty fun.