Is the reward for completing the Quran written for those who participate in a Quranic completion (khatmah) through online forums by dividing the reading, and is there any sin, invalidity, or prohibition in that?

There is no sin in writing down the verses of Allah; rather, the one who does so will be rewarded if their intention is good. However, they will not receive the reward for completing the Quran, nor will they be considered a reciter of it merely by writing it. But if the writing is accompanied by the recitation of those verses, in a voice that one can hear themselves, then they will be rewarded for what they have recited from the verses.