Can I apply for an ED visa at Chaeng Wattana if my visa-exempt stamp has less than 15 days left?

No, you cannot. At Chaengwattna, you must have 15 days or more remaining with your current stamp (visa-exempt or tourist visa). Now that you don't have enough days remaining, you must apply for a 30-day tourist extension first at the IT Square Laksi Plaza office, and then you are eligible to apply for an initial in-country visa at the Chaengwattana office. You need to check with them whether you can apply for a tourist extension and an initial in-country visa on the same day or not, in your case.