I do not think it can be the star of St. Stanislas. Leaving aside the color, when a badge worn normally on the sash moved to the neck, it was no longer accompanied by a star. That applied not only to St. Stanislas and other Russian orders, but was a general principle, accepted in other countries as well, to mention the Prussian Red Eagle as an example.
Besides that of the Order of the White Eagle, the only other gold star I can think of was that of the Saxon Order of St. Henry. But for historical reasons it was conferred sparingly upon Russians and I am not sure any Russian general (well, except Barcalay de Tolly) ever received the 1st or 2nd Class (only these two classes came with a star).
I have never heard of St. Vladimir with crown. Besides, in the times of Nicholas I only the highest class of an order reveived could be worn. That the cross with crown is of the Polish honorary Decoration is something I am absoluely positive about. With its black enamel I do not think it could be anything else.
Here is the list of the eight generals that can potentially be on the picture. Enjoy!
Sergey Baturin (Батурин, Сергей Герасимович)
Mikhail Cheodaev (Чеодаев, Михаил Иванович)
Alexandr Gerve (Жерве, Александр Карлович)
Pavel Pritvitz (Притвиц, Павел Карлович)
Karl Rennenkampf (Ренненкампф, Карл Павлович - this can be crossed out. I have found a picture of his and he looks entirely different)
Grigory Sergeev (Сергеев, Григорий Алексеевич)
Afanasy Tolmachov (Толмачёв, Афанасий Емельянович)
Alexandr Zaborinsky (Заборинский, Александр Никифорович)