Could ibs diarrhea form be the only symptom appearing including no skin involvement in diffuse scleroderma meaning no other symptoms but a positive ana with negative ena?

Quite doubtful.: In scleroderma, sacular diverticuli can form, but this is usually not early. In the absence of raynaud's syndrome, subungual capillary abnormalities (rheumatolgist use a 30x microscope to examine for this), and more specific serologies......I conclude your problem lies elsewhere.

Highly unlikely: Diffuse scleroderma refers to diffuse skin involvement. To have only internal manifestations of scleroderma but no skin involvement can occur rarely, but the bowel should show some manifestation of scleroderma such as bacterial overgrowth or sphincter incontinence, etc. IBS is normal bowel that is misbehaving. Tests such as colonoscopy, fecal fat analysis, ultrasound, etc. can rule out scleroderma