StudioLive mixers of all generations can be used as audio interfaces. The only 64 channel mixer currently available is the 64S, all other are 32 channel mixers and the 16R is a 16 channel mixer. The most effective and compact set is a SL 16 console (now SL 32SC) with a 32R rack mixer. The rack mixer has 32 mic/line inputs and 16+2 physical outputs for flex mixes and Main l/r. The SL 16/SL 32SC has 16 faders, but 32 input channels, which are handled on two "layers". You can reduce the numbers of faders by using the User Layer or DCA groups. All consoles have these features.
It says in the descriptions that the mixers now have 64x64 USB and AVB matrixes, which is true only for AVB. USB on the 32 channel mixers is 48x48, because the mixers are simply not designed to handle more streams. Only the 64S as quad-core mixer offers the full spectrum. The 16R as single-core mixer has even more reduced capabilties for USB and AVB.
Using Studio One and DAW Mode are two different things. The templates for the songs organize a 1:1 routing (channel 1 -> track 1 -> channel 1), which "inserts" Studio One to the processing and mixing done one the StudioLive console. The purpose of the templates is to make fast recording and playback possible. Just switch the input sources of the console to USB, then you can listen to the recorded tracks.
DAW Mode means shifting all activities to Studio One and using the StudioLive console as physical interface (faders, encoders, scrible strips) for Studio One. All processing is happening in Studio One, the faders simply represent what is done in Studio One. It always takes some time to understand the differences. If Studio One is the heart of the System, you will be using DAW Mode. If the mixer is the heart, Studio One is the "second heart".
Hope this helps. The mixers and Studio One are meant to work together and it's really fun to explore this universe!