The core of the question lies in understanding why there is a necessity to fix the output_1 amount to a "dust" level when adjusting the covenant input index to 0, seemingly creating an environment where the second output's quantity could be set freely.

Exploring the structure of transactions, it's highlighted that there are two inputs and two outputs involved in this scenario. The relationship between these elements is defined by setting input_0 = output_0, with the covenant input index being fixed at 0. This setup implies a direct correlation between the first input and the first output, suggesting that their values are intended to mirror each other. Consequently, this configuration inherently imposes a restriction on the second output (output_1), dictating that its value must be equivalent to the amount of input_1 minus any applicable transaction fees.

The essence of the query is to dissect the rationale behind limiting the adjustability of output_1's value, particularly in light of the apparent flexibility offered by fixing the covenant input index at 0. It seeks to understand the underlying principles or constraints that necessitate this particular arrangement, especially considering the potential implications it has on the overall dynamics of the transaction process.