Will God reunite me with my deceased friend in the Hereafter in reconciliation if I forgive him for not accepting my reconciliation, and I prayed for his well-being? And will God forgive me for my lie in which I accused a girl of zina (fornication/adultery) (without verification) with the intention of keeping her away from my friend and protecting him from sins and evil deeds, even though she was causing him to doubt religion and the Creator?

What you mentioned indicates the danger of bad companions. For repentance, one must distance themselves from environments of sin and strive for good environments and circles of knowledge, just as the man who killed a hundred souls did. The repentant person must leave places of sin and friends who aid in it, and replace them with the company of good and righteous people. As for your prayers for your friend's well-being, we hope they will benefit him. In Paradise, Allah will remove malice and grudges from the hearts of its inhabitants, and they will gather as loving brothers. As for your lying to the girl and about her relationship with the young man, it is forbidden, and you must repent and seek forgiveness for them, and remind those who slandered them of what contradicts what you mentioned of backbiting and slander, and seek forgiveness for them.