I have not seen the tennis ball before
but if she works what the heck. Many of the aftermarket tanks/cells have internal check valves built into the return bung. Some also have a roll over check valve or ball in the fill neck. To verify simply remove hose and look at tank bung for a ball or spring loaded valve. They are included with the cell we use from RCI.
Many of the hoses designed to go from cap to cell are conductive to remove spark dangers but the wire if redundant ground or not sounds like a good idea. Do not think tennis ball will work on the return line.
PS>>> I wanted to mention that the ground wire on your sender for the fuel gauge is isolated from the tank. The 2 wires that are on your sender for the gauge do not provide a ground for the tank body. If not secured to the frame in way that it is grounded please use an additional ground source.