Can Almighty God reject the repentance of a disbeliever who has committed acts of disbelief, given that he has repented sincerely and committed to prayer and acts of obedience, taking into account His saying: "Say to those who have disbelieved, if they cease, what has passed will be forgiven them"?

Allah accepts a servant's repentance if it is sincere and fulfills its conditions. This is purely out of His grace, glorified be He, not due to any merit on the part of the servant. Allah has, out of His grace and mercy, obligated Himself not to punish those who repent from their sins, for He has prescribed mercy upon Himself. As for the repentance of a disbeliever, it is accepted by consensus, and the sincere repentance of a sinner is certainly accepted by the majority of scholars. So there is no need to worry; your repentance is accepted, God willing.