In a permanent law

You mentioned Professor Koppel's explanation of fixed law. You explained that in his opinion the main distinction is between a concrete doubt, as in the piece you interpreted, and a theoretical doubt, the question being what would happen if it were to be withdrawn. You asked him 2 central questions. 1. Why is this not also true in the case of the Delita Kaman rifle? 2. Why is it not treated as a concrete doubt regarding that piece when bought from the store?
I understood his argument a little differently. In my opinion, the main question is whether we are discussing only a single piece or are we forced to discuss the entire complex (here this does indeed stem from the fact that the doubt does not refer to a specific piece) and this is reminiscent of the ruling of 2 paths.

In fact, a fixed law is built on several assumptions.
1. I am forced to discuss the entire complex because there is no separation.
2. The prohibition certainly exists within the group (Koppel mentions this as a required condition for a permanent law. This is explicit in my opinion, but I am not sure it is required in the Gemara)
3. There is no cancellation option here.
The result is that we are forced to decide about the entire complex, but this is not possible for reasons 2 and 3.
This is reminiscent of the ruling of two paths: if each can be discussed separately, then it is pure, but as soon as one is forced to discuss the whole, it is no longer possible because a counterproductive ruling is issued.
The question of why there is no fixed law in the case of the rabba dalit kaman is irrelevant because condition 2 is not met here. The first question of why not discuss the piece separately is still not so simple and it still needs to be explained that the time of the formation of doubt is being referred to as you mentioned.