one hot, steamy  Tuesday并不是后置定语，而是found的时间状语。没理由加逗号。

To easily find...为不定式作外置宾语。

我觉得不要加逗号。因为这里的 it 是形式宾语，后面的 one hot, steamy  Tuesday to easily find a seat in a car that was nearly empty 其实是真正的宾语。相当于说 so she found [one hot, steamy  Tuesday to easily find a seat in a car that was nearly empty] a paradox。

句子大意思是：西尔维亚经常乘坐拥挤的地铁在高峰时段去上班，于是她发现这是一个矛盾——在一个炎热的、让人气喘吁吁的星期二却很可以很轻松地在一辆几乎空着的汽车里找到座位。