Ok, for the Encryption unsuccessfull problem, this is what I did to get my Galaxy W to work.
In addition to format /system, /data etc, I had to format /storage/sdcard0 as EXT4. This gets past the Encryption unsuccessful problem, and I can flash the ROM CM12 and get it running.
But, I'm running into a weird problem that no one else seems to have come across. I can flash gapps 5.1.x, apparently without any failure messages on the flash window, and boot up.
Once the boot up is completed, I cannot see any of the Google apps icons at all !!?
I followed the same flashing process when installing CM 11 and 4.4 gapps, and this works fine. With CM 12, 5.1 gapps, I don't get any Google apps.
Anyone else came across this?
PS: After looking in the right place, many (all?) have the gapps problem in the cm-12.1-20150912-UNOFFICIAL-ancora thread. But I've also tried the RR rom with OTA updates. RR works - OTA just sits there with a blank page.