[MrPedantic]

Hello Nyugaton,

I'll divide the example into three parts:

1. It was performed at P. G., | the opera house named after the architect, | C. G.

The second part gives us more information about "PG" (it tells us the reason for its name). The third part gives us more information about the architect of the opera house (it tells us his name).

The comma before CG means that CG is in apposition to "architect" (that's the function of the comma).

Since no particular architect has been mentioned in the first part, the definite article before "architect" can't have the function of referring to a thing or person that has already been mentioned. It must therefore refer to a person or thing that is implicit in the context. In this context, the likeliest reference is therefore to the architect of the opera house.

This does not apply to the second sentence, however:

2. It was performed at P. G., the opera house named after the architect C. G.
Here, because there is no comma, "the architect" modifies CG: we know only that the opera house was named after him. He may have designed it too' but we can't tell from the sentence.

_____

That said, it is quite possible that #1 would be used with the intended sense of #2.

All the best,

MrP