I agree with you that presenters may be exaggerating how much stock has been sold to create time pressure and get viewers buying out of panic.
But that doesn't match up to Gemporia's self-described ethos of ethical selling. After all, if the items are of good quality and customers can make an informed decision to buy, why is there a need to push them into purchasing?
After all, customers purchasing having all available facts must be less likely to return goods and request refunds. Also, I can't see high end jewellers telling their customers that they only have 30 seconds to decide to buy. Or that prices will be going up in the next hour, or next day.
Gemporia isn't designed for people to take their time on deciding how to spend their money, and being very sure about what they want before purchasing. It's all about buying items on very little information, and being rushed into spending money without having the chance to think things through.
That's fine if you're just wanting to treat yourself to something inconsequential. Something really cheap and cheerful. But once you start getting into the hundreds of pounds it seems very risky.
As for stock allocation: nobody can guess in advance what is going to sell well. Which is why, as you said, sales may be exaggerated to try and get more people buying.
Spreading stock around the day's shifts can help catch more people, such as those who missed the earlier presentation due to work, doing the school run etc. Or those who only follow certain presenters and may have missed the previous presentation.