2:43AM
But overall, all three are geared towards song-making in a non-linear way, right?
For the most part, yes. The "Song" tab allows for linear construction of patterns, with the "limitation" of one pattern per engine. Within a host, you can run multiple instances if desired, but I have never seen this as a limitation. E.g., for a given kit on an engine, pattern 1 could be the Intro, pattern 2 the chorus, etc. 8 "full blown" kits/engines in a single song can become overbearing, so other engines/kits tend to have percussion or ancillary sound effects.
2:43AM
One key point that maybe I'm missing is that I still need to create sounds.
A few ideas here. I most often use Geist with SONAR as the host (need to clarify this), and Geist comes with a plugin called "Spitter" which will then be seen as an input on Geist's Sampler tab (top right). You can insert Spitter any place that will take a VST plugin, and Geist is limited to sampling one at a time. You can use this to sample any audio source (synth output, AD2, audio buss, etc.), so functions as "recording what you hear" for that track. From this, you can readily choose any sound source + other FX prior to Spitter (easiest to put it on a buss for this).
Regarding AD2 in particular, the AD2 engine buffers audio and keeps the last 15 seconds the engine plays (in the lower right corner). Although this can be useful, AD2 offloads each with the
same name, so this can get rather clunky. If it renamed them, it would be simple to just drag/drop them onto Geist pads; but if you have the patience to rename them, you can still use this and enable "slice" mode (lower left section) on Geist to allow you to slice that sample before committing it to the pad.
Geist allows imports of both wav and rx2 formats, so any sample in that format is "free game." If you choose to create your own, the "spitter" utility is very effective to pass audio into Geist's Sampler.