Is it permissible to delay breaking the fast after the Maghrib Adhan by about 20 minutes, with the intention of maintaining the first row [in prayer] and concealing one's fasting from people?

Hastening to break the fast is a Sunnah, not an obligation. There is no harm in delaying it for that duration. However, it is better to combine hastening to break the fast, in imitation of the Prophet, peace and blessings be upon him, with preserving the first row. If you fear showing off, then strive for sincerity and do not abandon the Sunnah out of fear of people. For, being mindful of people leads to losing many opportunities for good, and abandoning an action for the sake of people is showing off.