Zikr & Dua

Using the plural form when making dua in congregation

Q: When a person is reciting masnoon duas loudly wherupon others say "amin" (for example the imam after salah, at the time of nikah, etc), should he change the pronouns in the duas to plural form? For example, should he say "rabbi zidnA 'ilma" since he is making dua for the whole congregation? What would be the plural form of the dua "Rabb-irhamhuma kama rabbayaani sagheera"?

Making zikr

Q: My mother practices many zikars x-number of times as she’s been told by her late-brother that this brings much success. He was involved in Tableegh for MANY decades. I am trying to persuade my mother to abandon all this, and to solely act upon the authentic Sunnah. A close friend who is closely involved in Tabligh said to me that for me to abandon such practices is a sign of nifaqh, and as good as the Sunnah as it has supporting evidence from the Sunnah, that I am ignorant of. My questions: Is he correct? It’s a sign of nifaqh? Is it sinful for my mother to abandon all such practices, and solely rely on the Sunnah.