1.这是句子的主干部分：The ...approach...had been based on payments data...,
即：.方法是基于.数据的.
The bank’s mark-to-model 和 to valuing its subprime-related holdings 修饰approach,就是说,那是“银行估量其与次级债相关的股票所用的按模型定价方法”；其中 mark-to-model 的意思是“按模型定价”,而valuing its subprime-related holdings 的意思是“估量其与次级债相关的股票”.
payments data from the underlying mortgage loans.的意思是“第一担保抵押贷款的支付数据”,from the underlying mortgage loans 做 payment data的定语.