Mask better than green screen - alpha channel?

Hi,

I have have 2 items, 'front' and 'back', where front is partially obscuring the back.

I want to keep the front image, but use differnt back images.

If this was a simple colid colour, then colour/ultra key with the new items behind would be easy.

If i have the back item as green to cololur key that then show any item, the way i have lighting and shadow, its doesn remove that so not a clear mask. 

I think instead of green screen, i could use an alpha mask if i coudl create on myself. is, i coudl have the items to keep in white, item to remove/be replaced in black, and if i coudl create this and import as the alpha map for a video, then that would remove the area i dont want, and in another layer can put in new items i do want. 

Hopefully that makes sense.

Summary:

1. dont know how to remove shadow off eg a green screen item

2. think alpha could be used and can easily make my own, but cannot see how i can import my own alpha channel and use that instead of the current alpha in a video.

many thanks