Can you move the cup holder to the front??
Yes you can , you can face it to the sides or the front for both the cup holder and the headphones.
Is the 55inch vs the 60 inch how big the work space is on the desk?
Plenty of room and i have a lot of stuff for my desk. I have a full size printer, a keyboard. Mouse, gaming headphones with a stand, microphone, lamp with a pen cup , west rest pad and still have plenty of room to work.
Is there a California proposition 65 warning for this product?
Not that I know of.
Could 2 monitor stands be purchased and used on the desk?
If the monitors aren’t huge than yes , the desk is pretty big and you don’t have to add the shelf on if you can’t fit it ! It’s removable