To provide more information on the 18i20 working or not question, I got a response from Focusrite support regarding using the Focusrite Scarlett 18i20 with Akai MPC One. (emphasis mine)
We would also like to inform you that after looking through some of our ticket history and discussions with some fellow engineers here, the AKAI USB hosts on the MPC One and other devices, have been problematic to some degree for more than a few users.
Please reach out to Akai for further help or to submit a support ticket to them to address this issue on their hardware.
In other words, Focusrite have received multiple support requests or problem reports from people trying to use their products with "AKAI USB hosts". Though it is probable that they don't know what they're talking about. It was VERY difficult to explain to the Focusrite support what this is about, and in the end I don't think they
really understood that the Akai standalone box IS the computer here. They said things like: (emphasis theirs)
We do see USB ports on the Akai however, please do not connect the 18i20 audio interfaces this way.