The highlighted "は" looks to be there for thematic reasons?
The highlighted "ga" looks to be there for neutral description?
The only reason が is used here is because 彼女 is the subject of a relative clause. Relative clauses don't have topics, so が is used instead of は.
In a main clause, the が in 彼女が would likely be exhaustive rather than neutral (because it would be weird to have a neutral が attached to something already in the "universe of discourse"), but here in a relative clause that's not necessarily true.
The は, on the other hand, is a plain thematic は. It's a very common construction to have the topic は follow a nominalization:
It is good that you have quit smoking.
In other words, は is used here simply because there's no reason to use が.
The GA you have highlighted makes the second sentence begin as: The THING that she was fixating on // obsessing about was...
Actually the key to unlocking this is the NOHA (NO WA) because こだわったの is a noun-ification of こだわった.
こだわったの refers to a noun now. It is ...whatever she was obsessing over.