This is the first version that I'm actually happy with.

Features

Automatically detects added/removed devices
Device whitelist/blacklist based on /dev/input paths
Assign different buttons to different swipe modes e.g. 276 for 2-finger swipe, 277 for 3-finger swipe

To do

Make the error handling a bit more lenient

Right now, if the code fails to grab the mouse cursor, it will fail with an error. In practice, this isn't a fatal error, so it shouldn't be treated as one.
Automatically determine which resolution to use for the virtual trackpad

Is there a formula or equation to use for the resolution? It depends on the actual source device, right?
Allow whitelisting/blacklisting based on device names, product/vendor IDs, etc.

/dev/input paths are ephemeral. Ideally you should be able to configure which device you want to use in some persistent way.