My daughter is the one who flared after dropping mtx from her Remicade + mtx regimen. Her doctor checked her Remicade levels during the flare, and her levels were low and she had just started to develop Remicade antibodies. We put her back on mtx, but also raised her Remicade dose at the same time, and now she's been good again for a year, with sufficient Remicade levels and no antibodies.
I'm pretty confident that in my daughter's case, mtx acts as a boost to her Remicade levels. What I'm not so sure about is whether or not we could have gotten the same good results by just raising her Remicade levels up more without having to add back mtx. Crohnsinct's daughter's new GI believes this--that as long as you keep Remicade levels high enough, patients won't form antibodies.
I don't know which is better for long term health: using mtx + Remicade or just using a higher Remicade dose with no mtx.
In your daughter's case, if she does decide to drop Imuran, I would try to get her doctor to test her Remicade levels several times in the 6 months after dropping it, to make sure that her levels stay therapeutic and that antibodies aren't developing. She wants to find this out early, before the antibody levels get so high that she can't use Remicade any more. If she does see dropping levels, she might need to decide whether to add back an immunomodulator (which could be mtx), or just raise Remicade dose or frequency.