Hi Michael,
I’m just working on a similar project myself. We shot a person on green screen, interacting with a fictional coffee machine in a fictional kitchen environment. (image attached)

The environment and coffee machine were all created in 3D in maya, rendered out as still background images and imported into AE for compositing.
We didn’t actually have the actor holding things, but they were pressing fictional buttons on the fictional coffee machine.
We made all the props out of MDF and painted them green. (we took a sample of the green screen background down to a diy store and got them to paint match it).
One problem we encountered was that we drew tracking points in pencil onto the objects to represent the buttons. Now when we key, we’re having to roto out these pencil marks as the hand passes over them, simply because they’re not green!
My advice to you would be to use actual green props – so use two pieces of broom handle painted green with some green string or paper in between to simulate your scroll. (or something similar) This can all be keyed out and the hands will look like they’ve got the correct grip to insert your futuristic device.
Also as i think you’re doing, test test test – go through the whole post production process with your footage to try and discover problems…..we did some testing for our shoot, but missed this little problem, which is now causing me hassle!
Cheers,
Rob
(back to the roto!)