hello George and welcome on here
You're a very kind soul taking in the little waif queen. It is a shame about the woman who could not afford to look after her properly - she sounds as if she was just poor and ignorant rather than actually evil but enough of other humans.
You seem to know what to expect in many ways and I would not presume to tell you things you already know but please forgive me in case I inadvertently tread on old ground for you. I realise that you breed and show dogs so some of it may be familiar to you - the only difference is that this is a feline.
The dam will start to drip a bit of clear liquid around the time when she will soon deliver her kittens. As you have already provided a nice cosy nest for her, she will be fine - it is often said that the gestation period is 63-65 days but in my own experience - limited as it is - my queens have tended to err on the fashionably late side and have kept me waiting for as long as 69 days - mine are meezers though. I think you should take vetty's advice - kittens might appear any time between now and Day 68. If kittens have not yet appeared and your little princess has not shown any kind of discharge to indicate that kittens are arriving, then sometimes, as you are taking her to the vet, the journey alone will be enough to start things moving.
Hope things go well for you and the little queen-let.
best purrs