Is it permissible to forgo performing the absolute supererogatory prayers by sitting in the mosque awaiting the congregational prayer while reciting Surat Al-Ikhlas, and consider that sufficient as a substitute for the supererogatory prayers?

Sitting in the mosque to await prayer is a great act of worship. The angels seek forgiveness for the one who awaits prayer as long as he remains in his prayer place and does not invalidate his ablution. He is considered to be in prayer as long as the prayer keeps him there. This includes awaiting the prayer time or the establishment of the prayer. Reciting Surat Al-Ikhlas (definition) is equivalent to one-third of the Quran, but that does not compensate for prayer, as prayer is the best physical act of worship because it engages all limbs in devotion.