Are we rewarded for dhikr in the mind of the type of saying 1000 times estagfirullah, la ilahe ilallah, Alhamdulilah, ja mukal libel kulubi thebit kalbi ala dinik in the mind only without uttering a single sound or word. In other word you say them only in the mind without moving the lips. Does Allah accept and reward an invocation or supplication of the type: Allah please give me good understanding of quran and sunnah, Allah please give me strong iman, Allah please give me good health and solve my problems, if it is done in the mind only without moving the lips, without uttering any word or sound.
Every moment spent in the remembrance of our Lord is rewarded. You don't have to say it out loud to get reward.
There is the Hadith that tells Muslims to stop oppression when they see it. They should stop it with force if they can. If they can't use force, they should stop it with their words. If they can't say something against it out loud, then know it in their hearts that it is wrong.
If knowing that a sin is wrong in your heart is acceptable, why not dhikr and supplication?