1) Housing routing on the bars is a personal preference.
2) Are you saying that on your previous group the barrel adjuster was up at the brifter? There are usually housing stops on the down tube and on many road bikes the barrel adjuster is there. I just built up a bike with 6700 CD and it works there. You might need a different barrel adjuster.
3) The circumference of a 28 is larger and a 24 so the chain length from a standard or triple setup needs to be 2 links longer or the derailleur travel will not work correctly. But if you are changing large ring on the crankset from a 52 in the front to a 50 (typical compact setup) it is probably even.