Don't rule out the armenian or christian possibility. Stambol reminds me of Stamboulian, name taken by armenian living in Stamboul, a place which then formed part of today's Istanbul but was populated with Christian minorities, especially armenians. Zakar could be arabic but could also be a diminutive of Zakarian. Najar can be Syrian/Lebanese (I know several Najar and Najjar in Beyrouth, including prominent lawyers) but it could also come from Najarian/Nazarian. I don't want to take you on a wrong path but just try to help.