In the Name of Allah, the Inspirer of Truth.
Assalamu Alaikum Warahmatullahi Wabarakatuh

1. Sajda Sahw does not become Waajib, however reading a surah in the last 2 tak'aats of Fardh Salaah is itself contrary to Sunnah. One should avoid it.

2. Fardh and Waajib are very close to each other, in being an obligatory act. The difference is that Fardh has stronger evidence behind it than a waajib. To the extent that denial of a fardh can lead one to Kufr. If one neglects a fardh (when it is applicable to one) is a major sin.

And Allah knows Best
Wa Alaykumussalaam Wa Rahmatullahi Wa Barakatuh
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