Thank you everyone for your replies. Motorola, no matter how I approach them, seem to refuse to help me and I know why... I've now contacted Carphone Warehouse which is where I bought the phone. Not only am I having memory issues, I can no longer set a lockscreen, receive or view notifications, use the home button or pull out the 'back/home/multitasking' buttons from something that goes fullscreen. Misbehaving software is under their warranty, so they should be obliged to fix it right? I'm firing out all 'official' options before I start dealing with it manually. I've read thoroughly into the risks of flashing etc and it's something I'd much rather avoid doing...I'm not the average joe user, but I'm also not nearly as well versed into that stuff as you guys probably are.
I knew full well that once I upgraded to Marshmallow, I would not be able to roll back easily. All I saw on SD cards for Marshmallow was the adoptable storage option. I decided I wouldn't opt for it in the end and would just keep it portable, so I 'upgraded'. However, something specific to Motorola devices it seems was apps2sd, which is removed in Marshmallow. Now any app data on my SD is stuck in limbo and I can't do anything until this is all solved. If I knew this from the beginning then I would not have upgraded. David is right in that they should've notified users of this on the prompt screen, not buried it in the festering depths of Motorola's website.
So yeah, since I created this thread my poor poor phone that has been TAINTED by this wretched Marshmallow is even more unstable now. Any more tips on what I should do? Beginning to panic...