In Appian 7.3, when you have a list of one CDT nested inside another CDT, is it

To clarify, you're saying you have a CDT that is multiple inside another CDT? Are you trying to access all indexes of that nested CDT, or just a specific one?

Correct, a CDT that is multiple inside another CDT. I would be fine updating all of them or a single one. I just cannot see a way to store a value in, for example, pv!cdtHeader[1].cdtLine[2].bApprovedFlag...

As an example, let's say the CDT structure follows something like this:
{label:1, value:3, multiples:{{field1:2, field2:4, field3:6},{field1:1, field2:3, field3:5},{field1:8, field2:9, field3:10}}, info:8}

And this is all stored in pv!CDT. If you want to access one of the nested multiple CDT's fields, then you would use this:
pv!CDT.multiples[2].field2

This will return 3, the value of field2 at the second nested multiple CDT. From there, you can update the value as you want.