If I plug the 18 channel snake, would this also work as an 18 channel expansion at a live sshow?
Depends on the console.
A Qu16 is limited to 16x mono +3x stereo channels. Sources for those 16x mono +3x stereo channels can come from your choice of local preamps, Qu drive or the USB interface, but regardless of where they come from, the console can only process 16+3 at any one time. By default they come from the local preamps. Choosing to have ch5 source its signal from a USB input comes at the expense of ch5 sourcing its signal from the local preamp#5, ie the mic plugged into preamp#5 is no longer part of the mix: you’re still limited to 16+3. Adding a digital snake (not sure where you’re getting 18 from, it’s either 24 with the AR2412, 16 with the AB16, or 8 with the AR84) just adds another location (remote preamps connected via ethernet) from which the 16+3 can source their signals. It doesn’t add a whole lot of power to the console to be able to process an EXTRA 16 (or 18?) signals. In theory you could have 16(local preamps) + 3(local line ins) + 18(qu drive returns) + 24(USB returns) + 24(AR2412) + 16(AB16) = 104 input signals connect to the Qu16 at once… however you’ll only be able to mix 16+3 of them at a time.
In a similar way, the Qu24 is limited to 24+3, but it’s up to you where you source those 24+3 signals: local preamps, remote preamps, Qu-drive or USB interface. Whatever combination you choose, the console can only process 24+3 of them at a time. Qu32 is the same but limited to 32+3.
Where it gets interesting is the QuPac and QuSB options. These consoles have the DSP for 32+3 (like the Qu32,) however unlike the Qu32 they don’t have 32x local preamps. In this way adding a stagebox adds to the console’s input pool like in every other scenario, but also seemingly “unlocks” an extra 16 channels of processing. In fact these extra 16 channels were there to start with, they just weren’t being fed with preamps. It’s important to think of “inputs”"sources" and “channels” as separate things.
One benefit to having more sources than channels is if you want to connect multiple bands - then you can just reload a scene to repatch the desk between the bands.