There is a difference between a liquid diet and EEN. A liquid diet may help alleviate symptoms when dealing with a flare but is unlikely to provide all necessary nutrients/vitamins. Following EEN for 6-8 weeks, with a nutritional formula, will provide necessary nutrition, allows for bowel rest and helps mucosal healing.
My son was diagnosed just before turning 17. He did 6 weeks of EEN, using an NG tube, to induce remission. This worked for him. Without going into the full story but to give you 'background', my son then did supplemental/partial EN to maintain remission for two years. This kept him in clinical remission (no outward symptoms) but did not maintain biologic remission (ie without inflammation on the inside). Upon transfer to an adult GI, remicade was added. When diagnosed, he had inflammation in his small bowel, terminal ileum, some in large bowel and in duodenum - we were never given mild, moderate or severe classification. At diagnosis, he was also put on flagyl, thru IV for one week.
EEN (and possible flagyl ??) took away his symptoms immediately and took his inflammatory markers (CRP, ESR) down to normal levels. (They gradually rose as he reintroduced normal diet with partial EN as only treatment.) While on EEN, he was allowed clear fluids only - clear broth, jello, freezies, apple juice, etc. To accommodate school (and a teen's desire to not stand out), I sent him broth in a thermos so he could 'eat' lunch with friends and arranged for school to keep freezies on hand for him.
He learned to insert the NG very quickly and literally took him only seconds in the evening. He would insert it before bed, ingest the formula overnight and remove the tube in the morning. Elemental formulas are the most easily absorbed (allow greatest bowel rest) but taste the worst (not drinkable). My son used Tolerex.
If your daughter agrees to EEN, it would certainly give the humira a boost in getting her into remission.
I can't answer your question re pred as I have no experience with it. But, from what I've read here, pred can work quite quickly. I think it is possible that the pred has kicked in??? My son was diagnosed with fevers, diarrhea, nausea, night sweats, canker sores... all went away immediately, within days of EEN and flagyl... so, I'm thinking pred could have had the same impact?? But, I'm sure others with pred experience will jump in...
Good luck! I hope EEN, pred or humira will quickly begin to move her into remission.