Will this work if it hangs off counter top when extended?
No. It is only stable when it is fully on the counter top.
Can you use the stand mixer on the tray or do you have to take it off the tray to protect the wheels?
We have our coffee maker on the slide tray. Works well. Would work with a utensil not moving. If movement it would move the stand.
Is it safe to use on granite no wheel marks or grooves over time?
I also have Granite and it does not leave any marks or divits.