Goodness me this threade has goner off into flights of fantasy and will only serve to worry the OP into a nervous breakdown.
If any parent who ever hit a child were to be refused access I suspect that very few of us over 40s would ever see either of our parents again, plus OP has stated that XP is an ex heroin addict and has no proof of the other issues.
Drink/drive limits in France are much lower than in the UK and although the chance of being stopped is less, the penalties are harsher. If OP was that worried I am sure she will ask the Police to breathalyse the ex.
It may well be that mobile coverage at the new GFs house is rubbish, as it is in many places in France and the UK, or that DS is having such a great time that his phone is in the bottom of his bag only to be dragged out when he wants to use it.
Is DS does not often phone his Dad when at yours, he probably doesn't think to phone you when he is at Dads.
If XP had said we are going to "y" village with no mobile coverage you would not be expecting a call, the fact that the actualite does not measure up to your expectations is not a cause for concern, I would not be worried until, as you say getting on for DS's bedtime tonight.
(Mum of 2 and step Mum of 3 more so have some experience in this matter)