Blank (i): reduced to
- Context: The new school of thought is taking a complex, mysterious concept ("innate talent," "divine blessing") and explaining it away by simpler, observable factors (practice hours). This process of simplification is often described as reduction.
- Fit: Reduced to means to simplify or bring a complex idea down to its most basic elements. This perfectly captures the argument that genius is not mystical, but simply the result of practice.
- Misattributed to suggests the explanation is wrong, but the sentence is presenting this as the new correct theory.
- Measured by suggests a metric, but the core action is one of simplification and redefinition.

Blank (ii): quantifiable
- Context: The factors used for the explanation must contrast with the "divine blessing." The factors are exemplified by "thousands upon thousands of hours of grueling practice."
- Fit: The hours of practice are concrete, measurable, and countable. Quantifiable means capable of being measured or expressed as a quantity. This contrasts strongly with the idea of an unmeasurable, divine gift.
- Intrusive (disruptive) and pervasive (widespread) do not logically relate to the core concept of using practice hours as an explanatory factor.

The completed sentence reads:
A new school of thought has it that innate talent can be conveniently reduced to a series of readily quantifiable factors - Mozart's genius then is no divine blessing of the type conferred on a select few, but is simply the result of a patriarchal father who stressed, above else, thousands upon thousands of hours of grueling practice.