What is the meaning of a Muslim’s deeds being invalidated, and do sins committed in solitude expel a person from Tawhid (monotheism)? Do those who commit such sins have intercession, and are their righteous deeds performed before these sins accounted for if they seek forgiveness afterwards?

The meaning of "frustration of deeds" (إحباط العمل) is their annulment and the removal of their reward. It is of two types: 1. Complete frustration by shirk (polytheism). 2. Partial frustration by sins.

Sins committed in solitude do not remove their perpetrator from the fold of Tawhid (monotheism) unless they constitute kufr (disbelief). Intercession includes whoever dies without associating anything with Allah, whether they had sins committed in solitude or not. Whoever repents from their sin committed in solitude, Allah accepts their repentance. As for whoever does not repent, their matter is with Allah if their evil deeds outweigh their good ones.