Does the prayer of saluting the mosque become void if the worshiper returns the greeting of Salam before performing it, and is it permissible to speak during the Friday sermon to enjoin others to do good and forbid evil?

If you returned the greeting, being ignorant of the ruling, during the Friday sermon, then there is no sin upon you. Otherwise, seek Allah's forgiveness and do not repeat such an act. Whoever speaks during the sermon while hearing the preacher's voice should be gestured to be silent and advised after the sermon. It is stated in Al-Mughni: Do not stop the speaker with words, but rather gesture to him. As for those who do not hear the sermon due to their distance, there are two opinions concerning them: the permissibility of speaking or its prohibition. The most correct view is that both opinions exist, and in the case of permissibility, it is recommended for them to engage in remembrance of Allah (dhikr) and recitation of the Quran.