You're right, there are multiple ways to solve this. But, I wouldn't recommend an aggregate in this particular case for the simple reason that it adds the complication of creating the aggregate without any corresponding benefit. Also, I've had clock drift problems using an aggregate when regularly turning off one of the components.
The big advantage to using a line mixer is it gives you physical knobs for adjusting the relative levels of the playback and FM3 audio. If someone doesn't have room on their desktop for that inexpensive and tiny mixer I referenced above (hard to believe
), then use the option where you connect the FM3 to the Solo. This does not add any conversions or buffering. There is a single D/A conversion for the output no matter which solution you choose (the Solo monitor path is analog). Nor does it have additional DAW I/O configuration steps. However, this option avoids the need to create and manage an aggregate. For someone who is unfamiliar with aggregates, like the OP, that can be a significant advantage.
The only advantage I can see to an aggregate is if the OP lives somewhere where he cannot obtain audio cables. However, the bottom line remains, as you say, there are multiple ways to solve this and I'm sure he'll get it working no matter which option he chooses.