Regarding the 12mm I had the following focus issue but am quite settled with it now. Focus -peeking works quite well on the XE2.
I find the inner ring cap to be quite loose on it unless that is because I have got it mixed up with one from a Fuji which isn't quite as tight.
Interesting- I rarely stop mine down, only the very corners seem to sharpen appreciably when stopped down, and they're not bad at all to begin with.
I will admit the infinity mark on mine is slightly off though- quote common with Samyang. I'd like to adjust the ring markings, but a sharpie and a couple of test shots is OK for the moment.