Your third example isn't valid. It needs a bit of tweak.
Tarou ga Noriko wo mita toshokan
The particle "ga" and "wa" both introduce a topic. But in a phrase to explain a noun, we use "ga" exclusively. Your main point still holds, in a sense that "Tarou ga Noriko wo mita" is a valid sentence. But to be precise, "mita" in those two sentences are different conjugated forms; it just happens that two conjugated forms are the same in the verb "miru" (to see).
Tarou ga Noriko wo mita toshokan
The particle "ga" and "wa" both introduce a topic. But in a phrase to explain a noun, we use "ga" exclusively. Your main point still holds, in a sense that "Tarou ga Noriko wo mita" is a valid sentence. But to be precise, "mita" in those two sentences are different conjugated forms; it just happens that two conjugated forms are the same in the verb "miru" (to see).