How is it conceivable that some Companions would abandon eight rak'ahs, despite knowing the virtue of Duha prayer and the building of houses in Paradise for those who observe it, given that they were keen on building houses in Paradise by observing the regular Sunnah prayers?

The Companions did not adhere to a specific number of rak'ahs for the Duha prayer; they would pray two, four, six, or eight. This matter is broad. It has been reported that Sa'd ibn Abi Waqqas and Umm Salamah used to pray eight rak'ahs for Duha, Aisha six, Ali four, and Abdullah ibn Umar and Ad-Dahhak two. Each of them reported what they saw and heard, and it has not been established that the Prophet, peace and blessings be upon him, consistently prayed eight rak'ahs for the Duha prayer.