Effect of Remicade, pred., MTX, plaquenil, celebrex on fracture healing

This is a very difficult question to answer. Too, additional information about age, gender, and whether a diagnosis of osteoporosis is present would be important to the discussion. If osteoporosis is present, what is the T score, and what is the treatment, what is the vitamin D level?
I will try to give a general answer based on the available information. Plaquenil, methotrexate and Remicade should not have any ill-effect on the healing of a fracture. It is possible that prednisone has contributed to the presence of osteoprosis, which could delay fracture healing. That issue could be determined by obtaining a bone density test. The vitamin D level could be determined by a blood test. Low levels could contribute to slower healing. I am very doubtful that Celebrex would play any significant role in this case. There have been some reports that bone fusion in spinal surgery may be delayed by concomitant administration of non-steroidal anti-inflammatory drugs, such as Celebrex, and ibuprofen, but this has not been proved and I have doubt that this is the case.
So, in terms of your medication,that leaves us with some possible influence from prednisone. Some people feel that cigarette smoking can even delay healing. All that being said, nonunion of an ulna is not rare, even with no risk factors. Some angulations of a fracture are more difficult to heal than others. The short arm cast seems like a reasonable choice for you now. You could still end up needing surgery to heal the fracture. I hope all goes well.