Tow Truck Nick said the girl was asking about how to attach a seatbelt, but apparently, she was asking about attaching her own belt or belt loop to the seat belt - that is such a strange thing to ask?
If the above does show the seat belt (I assume the one that hangs from the roof for the middle seat), then that is strange too - as they wind back when released, so unless broken there is no reason why it would accidentally get trapped in the trunk/boot door surely?
I think if it was to be ruled foul play, like the case of Sarah Everard in the UK, when LE release their findings and detail all the evidence they actually have but never released, we could be in for a surprise!