The barrels have different diameters where the triple tree mounts. So either swap triple trees or see about turning them to correct diameter. The trigger packs are swappable between platforms but the parts are generrally not swappable between lowers.
There are more 922 parts for a G3 lower than a CETME and you have options to upgrade the performance you do not have with a CETME. G3 is better all around.
Buttstocks and backplates, you can get an HK backlplate assembly to interchange with a CETME if you use a cetme guide ring and end stop (sorry but proper term eluding me). The parts are not interchangeable between stock assemblies other than that.
2nd, pressing barrels. There are builder forums that can detail this. In a nutshell, drive out barrel pin, press barrel out of trunnion from rear to front using a press. Drop new barrel into trunnion, press most of way but short of flush, use your bolt carrier in the trunnion with an appropriate gap spacer between bolt and carrier and use press to press bolt carrier barrel to desired gap. SWEAR A LOT!!!, Then swear some more. This is point you want to read an actual tutorial to see the fine details. After desired gap achieved drill through the barrel pin hole of your trunnion and press your new barrel pin in place to hold the barrel at desired gap.
4th- I would say no on the chroming. You would be changing the tolerances on you parts and it could create issues.