With your condition, I understand how concerned you are. You're right! Your condition needs etensive follow up to arrive to a definite diagnosis.
So you have done MRI as well, we should wait what the result says then. The lymphocytes levels in leukemia are not always high. It is also possible to have low levels of lymphocytes count in leukemia. Now, don't think that I am saying you have leukemia. You need series and follow ups of investigation to arrive to diagnosis. There are many other possible causes that can affect the lymphocytes too which include use of corticosteroids, autoimmune diseases, infection or disorders of the lymphoid tissue in the spleen, lymph nodes, and thymus gland. Your should have immune tests as well. You have not mentioned bone marrow biopsy, if not done yet, I would highly advise to have it done.
As far as I know, Celiac does not directly cause low lymphocytes count but what I know is that in Celiac, the epithelium villi of the intestines are infiltrated with lymphocytes. But I think, they are running Celiac test to establish some association to your current condition.
Lastly I would say, NOT ALL causes of low white blood count or low lymphocytes are known in our current time. There are many cases similar to you that run all sorts of tests but found nothing and are therefore categorized as IDIOPATHIC for the time being, which means the condition is with UNKNOWN cause.
For now, bear with your doctors who are trying to find leads to your problem.