Can untreated Celiac disease cause Immune Reconstruction Syndrome? I have bad allergic symptoms without Montelukast. Untreated Celiac disease too.

Not known. The association of Celiac and immune reconstitution is not one I have heard of or could find doing a quick search. Montelukast is used in allergies so I think having the expected response from a medication does not suggest an immune problem. It is worth getting your Celiac under control for other reasons, i.e. to prevent the known Celiac complications.
Unclear. Do you mean immune reconstitution syndrome? If so, this is more specific to HIV/AIDs or other cases of severe immunosuppression then being "reversed" by initiation of treatment/reversal of immunosuppression. Hope this helps. .