Don't the 10 speed front derailleurs have a different cable pull to 11 speed?
Or was that just Duraace 7900?
Definitely need an 11 speed chain, you can use 11 speed chains on 10 speed, but not other way around (width of links)
Ultegra RX for the rear derailleur and ultegra 6800 shifters for the front for the CX bike? 11-34 105 cassette is pretty good value.