I do not use osx, but there is something called agregate devices you can create in the sound setup
section of macs, heard that somewhere when my son was setting up a macbook pro, you can also set
up the headphone jack as an input for whatever that small cable fits in there, you can also install focusrite
drivers instead of using core audio for your interface, you may be better off asking on a music forum about
which drivers work best for your audio interface, core audio or the manufacturers supplied ones, you will find
latency / buffer settings in the scarlet drivers to adjust for best throughput or latency.
Wish I could help more, but maybe someone who can will chime in.
Here is a link to using jack on osx,
https://www.youtube.com/watch?v=FhrJ9rTMcBU
I do not use osx, but there is something called agregate devices you can create in the sound setup
section of macs, heard that somewhere when my son was setting up a macbook pro, you can also set
up the headphone jack as an input for whatever that small cable fits in there, you can also install focusrite
drivers instead of using core audio for your interface, you may be better off asking on a music forum about
which drivers work best for your audio interface, core audio or the manufacturers supplied ones, you will find
latency / buffer settings in the scarlet drivers to adjust for best throughput or latency.
Wish I could help more, but maybe someone who can will chime in.