Just a heads-up on a 5200 controller experiment I ran last evening that was surprisingly successful for playing the 1 and 2 button games. (experimented using Space Invaders as well as Dreadnaught Factor).
This method is the 3rd unique way that's been discovered to get a mouse working on the 5200 digital games (not the pure analog games). Like the other 2 methods it tends to work on most of the 5200 game library (with varying results) with the exception of these six titles: Gorf, Kaboom, Missile Command, Super Breakout, Star Wars the Arcade Game, and Tempest.
The 2 items needed to play the single button 5200 games are a Masterplay Clone (mine was made by Atariguy1021 in 2016, and it has potentiometer adjusters which are a must for it to work right) and a Commodore 64 mouse (model 1351). This mouse is especially cool because it has the capability to simulate analog movement in digital games due to pulse technology that it has onboard. In other words, your Space Invaders ship can be moved slowly with slow mouse movements or more quickly with fast mouse movements.
To play the two button games you need two more items: a 9 pin splitter cable (two male plugs going to a female plug) and a Genesis 3 button gamepad.
The method to get things working for the single button 5200 games is as follows. Plug your Masterplay Clone into the 5200 and connect the 9 pin connector of the mouse to the port of the Masterplay Clone. Now before pressing the power button on the console, press and hold the right mouse button. Turn on the console and wait for the game to boot up before taking your finger off the right mouse button. The mouse should now be functioning properly.
For two button games connect the 9 pin splitter cable to the port of the Masterplay Clone (which you have plugged into the 5200). Connect a Genesis 3 button gamepad and the mouse to the male plugs of the splitter cable. Press and hold the mouse right button. Turn on the console and wait for the game to boot up before taking your finger off the mouse button.
One of the mouse buttons is your primary fire button and the second button is utilized by pressing the 'C' button on your Genesis gamepad. You will have two hands in use to play the two button games.
Well, there ya go. Games to try might include Ballblazer, Centipede, Dreadnaught Factor, Gyruss, Meteorites, Rescue on Fractalus, Space Invaders, and Zone Ranger. Have at it!