I apologise if the following answer seems odd,but it was something that just crossed my mind.I am 86 and my wife (Elsie) is 85,so we have had some (few thankfully,as we are in good health) dealings with Social Services.
Generally,Social Services will believe whatever the elderly person says(even if the person seems belwildered and not too "with it").
I say this because I wonder if your MIL has become vague,and what with her injuries the Social Services have decided that you (or other relatives) are abusing her?We know that this is wrong,but the Social Services don't,and they have only youe MIL's opinions to go on.
Think of the scenario:~
MIL has injuries.They ask how she got them.She mentions her Son "falling" on her.I am afrais that Social Services may have decided that she was " at risk" and prevented any family members from seeing her?
As I say we know that this is not the case,BUT if Social Services have added MIL's injuries to her account of how she(thinks) she got them that might be why they are "hiding her"
I hope this helps,by putting another train of thought in you head.My idea may be rubbish,BUT at least it's another road to go down?