magic, mam is quite unwell now, sadly. She did have a second round of treatment, but now she has too many brain tumours to treat (more than 30). However, hers are secondary to lung cancer, so hopefully this isn't any sort of indication of how things will go for your mum.
I think the consultant was right about the symptoms getting worse in the short term - that was certainly the case for mam. It is a very full-on treatment - mam's consultant kept emphasizing that it is brain surgery, albeit without actually opening up the skull, and it does take months to recover.
It sounds as though your mum has one (large-ish?) tumour, whereas my mother has multiple small tumours. Maybe your mum's doctors are hoping that they can knock her tumour out by whacking it with as much radiation as she can take, and possibly that's why they wouldn't advise a second round - too much radiation can cause brain damage in itself.
this is the website of the place where my mother had her treatment - it has some useful info.
It sounds tough on you and your mum - is it just the two of you attending appointments? Do you take a notebook to write down what is said? Otherwise the information can become overwhelming, especially when you're already stressed and frightened. Also, you can call the consultant's secretary and ask for a telephone appointment if you think of further questions once you're out of the room - it's a lot to take in.