This is probably a FAQ but I could not find the answer anywhere: since OG connects to the opener using the button terminals, it seems to replace them. Can I leave the button connected as well, essentially using the same terminal for both OG as the wall-mounted button keeping them both operational?
Yes, the only hassle is trying to get 2 wires into those tiny holes. But it can be done and it works.
The OG just has a relay that closes the connection in parallel with the wall buttons connection. There is no polarity to the OG wires so it does not matter which of its wires goes into which hole (among the two proper holes of course).
Yes, as jagos said: you can keep your existing button — in fact you can install as many buttons as you want and if any of them is pressed, it triggers the garage door action. You can think of OpenGarage as a WiFi-enabled door button.