Different error message this time w/ the new config... "AirPort_Ath****" was added before the usual Missing Bluetooth Controller Transport error. See attached screen shot.
I also forgot to say this earlier, but when I installed Yosemite, I also got the Missing Bluetooth error, but the install kept going. For El Capitan, after receiving the Bluetooth error, I get the "crossed out" sign and garbled text.
I don't know if it's an El Capitan installer issue because I used the same installer to update my 'real' iMac to El Capitan a few months ago.