I'm using the latest CM11 build by @cars1189. I don't remember what the difference between both kernels in cars' thread is, but I definitely know that the Christopher kernel is for OC and UV because it is the kernel i'm using right now. I will check it out again and come back at you when I find out.
For flashing one ROM over the other one I have done it many times (I don't know if it is fine or not but from the replies I think that it's not recommended) but i didn't encounter any trouble. Just but sure to nandroid before you flash on top. Also if it is an update of the same ROM you are running, it shouldn't cause any trouble in the future. The only thing is that if you had Xposed framework activated before the update, after the update you will have to reactivate it again because with an update of the ROM, Xposed framework gets deactivated. Also I would recommend a nandroid before the update just in case.
Also for the launcher, I have just found out that the home launcher of the Z3 has been ported and I tried it out. And I got to say, it is definitely faster than NovaLauncher in my phone. It takes less more time to go back when I press the home button from anywhere I'm in the phone. It doesn't have too much tweaks and personalisation like NovaLauncher, but hey with this phone the simpler the faster. The only down side with this launcher is that there is an empty spot under the dock that creates a waste of screen space. Here is one of my screenshot:
That's all I had to say, happy to help you if you have any other question that I can answer.