What happens if I spend most of the year outside Turkey after getting the certificate?

Turkish law does not use a 183-day test. As long as your domicile stays in Turkey you remain resident and keep the exemption, but the country you spend time in may treat you as its own resident and tax your worldwide income. If you move your domicile and spend no more than six months of a calendar year in Turkey, you stop being resident and your foreign income is outside Turkish tax anyway.