Phone loops on "Optimising apps". How do I disable the encryption so that I can mount /data and factory reset? (details inside)

Its a Galaxy S3 running modified cyanogenmod with android being on 5.1.1 I believe.

So, a while ago my phone shut down and refused to boot properly. While I still had liveboot working I managed to gather that for some reason it tried assigning new ID to packages (every single one of them), refreshing certificates and one other thing but then failed at every step because the file already existed. This, I assume is the same point where optimising apps hangs at.

Problem is, I had encryption enabled. Now, while the phone boots up to the screen where I enter the pin to unlock the phone, it DOES hang at optimising apps so I cannot access settings to disable the encryption so I could access /data in recovery and wipe it. Tried mounting and wiping data in recovery - nothing. Tried factory reset in recovery and it resulted in the same thing. Tried complete wipe while reflashing - no results. Pretty much the last things I can think of are ADB magic or something in Odin but I doubt that.

So I assume I have to get rid of the encryption to reset my phone, get rid of all packages so it stops looping and then reflash - but how do I manage the first step? Or, if the encryption doesn't stop me from accessing /data but something else is - how do I fix it?

Any help would be appreciated.