If a tower fails when the loading block assembly is placed (and the tower meets all dimensional and other requirements), does the tower receive the minimum mass (the Loading Block Assembly) and/or are they subject to tier 2-4?

The minimum load scored is that of the Loading Block Assembly (6.b, 5.b). So if a tower cannot support the Loading Block Assembly it must be scored in Tier 4; unable to be loaded. If the tower supports only the Loading Block Assembly, then the tower is scored in the appropriate tier (1, 2, 3) and the weight of the Loading Block Assembly is scored.