I've read accounts of Niacin working, but it didn't help my son at all - and he believes a lot of the same things as your mom.
If you want to try the Niacin, it won't hurt her if she's willing to take it. My son tried it and experienced a very uncomfortable hot flush and moderate doses. They literature explains that's what you will feel if you don't really need it. Maybe it will work for her?
The injections are great if you can convince her to try them. They've really helped a lot of people when oral meds failed. We started talking to our son about all the positives to a once-a-month shot early on - mostly convenience and how we would take one instead of our daily pills if we could.
He was still resistant and only agreed to try them once they were in the hospital and the doctor there talked to him more about it & he got the idea it would get him home faster.
It's not unusual for any med to give out. Sometimes, higher dosages work - other times, you need to switch to something new. The good news is that she'll take the meds - that's half the battle right there.
Another idea - how's her sleep? Sometimes, if the meds are working somewhat, and you can get them to sleep regular hours, it helps a lot. That's just something to try until you can get her back to the doctor.