Its not hard to find drugs in Brighton if you have money
True, but it is the rapid moving around combined with needing a constant supply of drugs, and the need to find a new dealer in each area rapidly. Not impossible, but it would certainly make moving rapidly from area to area much harder and less appealing if there were illegal drugs in the mix.
And how often do these chaotic lifestyle/neglect issues occur without there being any substance misuse issues at play?
I don't have statistics to hand, but I imagine they're almost always comorbid with something else - but not necessarily substance misuse. That something else could easily be mental health issues and / or coercive control.
For instance, if MG had MH issues that made him act irrationally and was also exerting coercive control over CM to ensure she acted alongside him. I wonder if, secondary to all this, CM also has a bit of a saviour complex; she knows he isn't well and thinks she's the only one who can help him (either through sheer love and willpower, or because he won't accept outside help).
It's also worth noting that coercive control is a form of domestic abuse, and domestic abuse can he the sole reason for removal in some cases.