How can these two noble verses be reconciled: "And he amassed (wealth) and collected it" and "And beneath it was a treasure belonging to them, and their father was righteous"?

There is no contradiction between the two verses. The verse in Surah Al-Ma'arij speaks of the warning for those who accumulate wealth and withhold God's right therein (i.e., do not pay its zakat and dues).

As for the treasure of the two youths mentioned in Surah Al-Kahf, it is not among the blameworthy treasures for two reasons:

1. That was in the law of those who came before us, where accumulating treasure was permissible for them, whereas it was forbidden for us (wealth on which zakat has not been paid).

2. The forbidden treasure is that on which zakat and God's rights have not been paid. The treasure mentioned for the two youths was lawful wealth for them.