What would happen if I used an output pin as an input? Let’s say I want to make double LED matrix. Since LEDs are diodes, I could basically put 2 LEDs per matrix cell and face them opposite directions. Like say Row 15 pin is +5v and Column 15 pin is 0V an LED will light, I then switch them Row 15 pin 0v and Column 15 pin +5v, the other LED lights. Will this damage the arduino? having +5v come into a pin set as an output?
I think you may be confusing direction of current flow and IO pin direction.
An IO pin can source current, as in when the IO pin = 5V and is connected to ground through a resistor, the current flows out of the IO pin to ground.
An IO pin can also sink current, as in when the IO pin = 0 connected to 5V through a resistor. In this case, the current flows through the resistor into the IO pin.
In both cases, the IO pin is configured as an output.
Take a look at [Charlieplexing - Complementary Drive. It sounds like it is about what you are trying to accomplish.](Charlieplexing - Wikipedia)