How could Jesus have given the disciples power and authority when he is yet to be exalted?
Actually, Jesus gave the disciples power and authority toward the beginning of his ministry, rather than at the end after he ascended to heaven. In Luke 9:1-6, Jesus sends out the twelve disciples and gives them power and authority over demons and to heal the sick.
They minister as they had seen him minister in his own ministry. They do the same things that Jesus does. Jesus had power and authority and could give it to whomever he willed at any point in his ministry.
Matthew 28:18 tells us that Jesus was given authority by the Father for the entire time that he was in ministry. One who has authority can give it to another. Nothing was stopping him from giving his authority to the disciples at any point throughout his ministry.
We also know that he ministered out of the power of the Spirit (Luke 4:1, 14, 18). So he operated in the power of the Spirit, and I am sure he taught the disciples this throughout his ministry. Later on the Day of Pentecost, the apostles receive the power of the Spirit when they are baptized (Acts 2:1-13).
By the time he ascended to heaven, he had already been to the Father. In the Gospel of John, he tells Mary not to touch him because he has not yet been to the Father. But by the time he sees the disciples later that day and in all the other appearances he makes post-resurrection, he has already been to heaven.
His ascension into heaven, or his exultation, was the last time the disciples would see him in person until his second coming in the end times. So there is nothing stopping Jesus from giving authority and power to the disciples. They certainly needed it more after he had left, and that is why he sent the Holy Spirit.