Static measurement would be by a Pro,to determne the duct systems overall resistance to air flow,and the ability to move air thru the ducts you added.
Testing would likely show that if you block the 2nd floor return ,to force the added returns to pull some air,the static woyld increase to the point that the fan can not move the required air flow,for the equipment to work properly and safely.
Simple test:Feel the air flow at supplies in Bedrooms,now block the 2nd floor return,feel the decrase in flow in the bedrooms,due to increased static.Same is tru to see if returns are needed,feel the supply with the door open,then closed the diference ,if any ,is the flow you should get ,if you add returns.
To get the returns you aded to work properly,they would need to be larger,and the duct going to the 2nd floor return,may need to be larger (may not have the space or access) as well,plus better fittings(no tees).
Your best bet is the passthru or jumper,returns unless you get a Pro to design the returns to be ducted .