Can I Reset My Firestick Without the Original Remote?

You can reset your Firestick by using a USB keyboard or mouse, or even through the Fire TV app. You can reset to Factory Defaults from Settings > My Fire TV.

How to Connect Firestick to WiFi after Factory Reset?

After doing the factory reset, your Firestick will be just like a new device booting up and it will ask you to connect it to WiFi. For that, you can simply use the Fire TV app or a USB keyboard/mouse to follow the setup process.