[Q] Boot loop when trying to encrypt on CM12 topic

Here's what I've done so far and info relating to my setup

Phone: klte - G900T (T-Mobile Samsung Galaxy S5). I'm not on SafeStrap, I'm installing to system. I'm running TWRP recovery 2.8.4.0.

I first installed the CyanogenMod 12 nightly for 20150204 with Paranoid Android GApps 5.0.1RC3. All good there. I then went to "Security" and clicked "Encrypt Phone." It didn't ask me to set a password, so I assumed it'd generate a random key for LUKS and encrypt it with the default password. All good there. It rebooted into the "Encryption Wizard," and went through encrypting the phone. Cool, I thought.

It then boot-looped on start, seemingly because it didn't yet understand how to decrypt using the default password. TWRP decrypted data on startup of the recovery just fine, so that's cool. Okay, so maybe the nightly doesn't support decrypting on boot yet. Fine. I upgraded to CM 12 20150208 (today's) and tried again. Nothing.

I then did a factory reset and wiped system, reinstalling CyanogenMod from scratch. No joy either, couldn't boot. What was weird was that I still noticed that it was mounting /dev/dm-0 at /data, so my data was still encrypted. Shoot. How to get rid of encryption? I wrote a ton of zeroes to the beginning of data to wipe out the LUKS header:

Code:

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dd if=/dev/zero of=/dev/block/platform/msm_sdcc.1/by-name/userdata bs=4096

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I terminated this process a while in (probably should have been more exact by specifying how much to zero, but laziness). I then ran

Code:

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make_ext4fs /dev/block/platform/msm_sdcc.1/by-name/userdata

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Next, reboot recovery, and all is well. Install ROM, and try encrypting again, this time setting an unlock code so that it actually uses a real encryption key instead of default. It reboots, takes a really long time, and then just continues into regular boot, no encrypting taking place, it seems to bypass that. Dangit.

I then tried switching off and on again the set_encrypted_filesystem flag to recovery:

Code:

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adb shell recovery --wipe_data --set_encrypted_filesystem=off
adb shell recovery --set_encrypted_filesystem on

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No joy, when I try to encrypt it still just reboots regular (albeit really slowly) into regular system and doesn't encrypt. Ugh, I feel like I've lost all progress. I've made sure to clear Dalvik and regular caches on each try, so things shouldn't be persisting. I had an old backup of the stock T-Mobile ROM, so I flashed back to that, and then tried installing CM 12 again and encrypting, and no matter what I do, I can't get to that encryption screen. $@##$!!!

What are my next steps? How can I see what happens in that boot, ie what fails, so I can fix it and get onto my lovely life outdoors?