Still booting into stock despite Slimrom being installed......

I'm at my wits end trying to root my wife's HTC One M8 - followed a guide but just hit problems at every step.
AAANYWAY, finally unlocked the thing (haven't done S-off), and confirmed it's rooted through root checker. Been trying to install Cyanogenmod, but I just get Error 7 in TWRP. Nothing seems to fix that.....got fed up and downloaded Slimrom instead. Eventually it stopped giving Error 7 for that when installing through TWRP and successfully installed. Rebooted and.....oh, I'm still on the stock ROM.

Okay, any ideas on what happened there? I haven't even had any luck in google.

Mysterious case of Insufficient Storage despite 5.08 GB of available space.

I'm using the Nobis NB09 tablet with Android 4.1.1. When attempting to download an app from the Google Play Store, I get Insufficient Storage error, despite 5.08 GB of available space and the app that I want to download is only 9.83 MB, so there is more than enough space to install it.

Available space reported in Storage:

Internal Storage

Available

66.24 MB

Internal Storage

Available

5.01 GB

Why is all this available space being ignored?
Why are there 2 sections for Internal Storage shown in Storage?