Is it considered a sinful act to delay breaking the fast in Ramadan until darkness falls, in order to be certain of sunset, due to not hearing the Maghrib Adhan? And is there a better way to verify the time for breaking the fast if hearing the Adhan is not possible?

There is no sin in what you are doing if you break your fast after fully verifying the sunset. However, you are not obligated to do so. Rather, you can rely on an approved calendar, such as the Umm al-Qura calendar, to determine the time of Maghrib and break your fast accordingly.