The judge hasn't given a sentencing date yet. I don't know why - usually they indicate that once the verdict is in. Perhaps there's a complication or perhaps reports/other considerations are needed.
The perjury charges will be dealt with seperately. If Maxwell is sensible she should just plead guilty because it's hard to see how any other conclusion can be reached. I assume the US has a similar system to the UK whereby an early guilty plea leads to a discount on your sentence.
If she doesn't plead guilty and it goes to trial, she'd simply be brought back from prison to face the charges and held in a local prison if she's not there already. The other option could be a video link, but, again, I don't know if the US offers that in the way the UK does.
It may also be that the judge is waiting to for an appeal from Maxwell's solicitors. Everything I've read suggests they have a steep hill to climb here. They would need to show that one of the judge's directions (eg the granting of anonymity to certain witnesses, but there are other examples) was firstly contrary to law and secondly affected the verdict which was reached. It's very hard to see how they can argue this, but I suppose, by the same token, Maxwell hasn't much to lose.
More than one agency is suggesting she really ought to have cut a plea deal at the start of this. That suggests to me that she does, indeed, know something truly damning. Perhaps she'll talk - it's hard to see, now, what incentive she has to remain silent. The Guardian suggests that if she talks, and has substantial evidence the plea deal could see her prison sentence cut to 10 years, and she'd be out in 7. If she doesn't, she's not going to see the light of day again.
I've mentioned this before, but I wonder if the judge has been asked to delay sentencing for a short time to try to allow a plea deal to be reached.