Is someone who commits riyaʾ (showing off) and then seeks God's forgiveness guilty of major shirk (polytheism), and does this apply to someone who says "May God guide us" after backbiting, or "I seek God's forgiveness" after cursing, without remembering their true intention, whether it was to please God or to seek praise or fear people?

You need to be aware of two matters: backbiting and cursing your nephew by his father, which is a reprehensible act due to the severe warnings established concerning reviling a Muslim. The matter becomes even more serious if the cursing is directed at a brother.

As for the phrases "May Allah guide us" or "I seek forgiveness from Allah," they are not considered hypocrisy (riya'). Rather, they are appropriate, and we hope they come sincerely from your heart. However, if one supplicates to Allah or seeks His forgiveness to please people, then that is hypocrisy.

Seeking forgiveness after hypocrisy is required and is not major disbelief, because hypocrisy is a sin, and seeking forgiveness erases the effects of sin, provided that the seeking of forgiveness is purely for the sake of Allah the Almighty.