If the Fajr congregational prayer has started, under what condition is a latecomer allowed to perform the two Fajr Sunnah units first?

Catching Obligatory Prayers Part 2: Sunnah Alignment

• In all seasons, without any condition.
• If they are certain that they can perform the Sunnah and still catch at least the final Tashahhud sitting with the congregation.
• Only if the Imam is reciting a long Surah.
• They are never allowed to pray Sunnah once congregation begins.