Perhaps the downfall in Bonsoir's debate is the fact she is taking her own single, unique and personal situation (which may work for her if the mother of her DSC did not have a problem in her "taking control" and becoming a surrogate) and generalising that beyond herself, to the circumstances in nicknames posts. And then, claiming it to be the "one and only version of the truth"?
It is clear for all to see that the SM is assuming responsibility that is not her's to take. OK, so for argument's sake, the DF could say "I am giving SM in loco parentis authorisation to do x, y, z. But morally and ethically, changing a hairstyle, especially on a little girl (Ie not a functional boy's haircut just above the collar, for example), ought to have some consultation with the mother. Otherwise it is not in the spirit of in loco parentis Where it becomes less of a reasonable, helpful action for the child's benefit and more about a power struggle and making a point, then it is not right.
nickname this is very stressful for you, undoubtedly an uphill struggle, nothing anyone can say (least of all bonsoir) can convince me otherwise that the SMs actions are beyond unreasonable, -- and "judgy" comments on this thread about neglect, add no value to you whatsoever.
Would it be best to take your situation forward to a counsellor who can give you some solid, practical and emotional advice tailored to your own needs with your DD. Taking any of this through a court, could haemorrhage cash from your budget and get you nowhere (and IME take "years off your life" ). Alternatively - and maybe check through that counselling session? -- a firm but constructive/ conciliatory letter to your xDH (maybe even via your solicitor, which I have found can be less inflammatory than a court summons, but effective to get the message home) that there are certain boundaries that need to be respected, and where collaborative consultation is absolutely critical to safeguard DDs home environment in a positive way.
Even though the general support you are getting here is helpful it is marred by differing personal opinion (she says, trying to be tactful
is just prodding at you and doing more harm than good, when your emotions are so raw and you need to find a way forward X