Notice the Lord does not touch Nephi. He speaks the words. The Lord's word is sovereign. If the Lord speaks it, it is so. It is not necessary for the Lord to lay hands on the servant He has just called, only that He speak the words of commission which give the servant "power."
Notice that it is "power" and not authority. It is the "power" to baptize "this people" which is granted Nephi. Why would "power" be required for a man to be able to baptize? What if the man possessed "authority" to baptize, but lacked any "power" in his priesthood? Is "authority" anything if it lacks "power?" What is the difference? Can a church spread about the "authority" to do ordinances if that church lacks "power" to do so?
Why are "that the rights of the priesthood are inseparably connected with the powers of heaven?" (D&C 121: 36.) If indeed all rights of the priesthood are inseparably connected with the powers of heaven, can a man who has never felt, experienced or had any connection with heaven hold any power? Hold any priesthood? What connection did Nephi have with heaven the instant the Lord spoke to Nephi the words: "I give unto you power"?
Why is it that "the powers of heaven cannot be controlled nor handled only upon the principles of righteousness?" What about ambitious men who view holding an office in The Church of Jesus Christ of Latter-day Saints as entitling them to direct, preside, control and dictate to others? What are the "principles of righteousness?"
Now, I ask those questions not to give people reason to rebel against those who preside over them. It is not for us to weigh, measure or respond with accusations against those in positions of authority. I feel a great sympathy and pray for them. However, I offer it as a self-governing, introspective question to anyone who has any calling, family position or power over another person. Whether it is in church, or at work, or in the family, or elsewhere, the way we deal with others ought to be informed by the same standards as use of priestly authority. But these things are for internal use, not as a measuring stick to be applied critically against others.
Often we are able to see clearly the errors of others, but are completely unable to see our own glaring errors. This is why I have said repeatedly that the Gospel is for internal application only, and not for external use in judging others.
In the case of Nephi, he already held power, did he not? He had preached the Gospel, used words having such power that listeners could not disbelieve them, raised his brother from the dead, and cast out devils. (3 Nephi 7: 17-19.) Despite all this, Nephi was called forward to receive from the Lord power to baptize? Why? Why if he already had such great power as to be able to raise the dead, did he need a new grant of power to baptize?
Does the possession of authority in one dispensation (Moses') continue into another dispensation (Meridian of Time)? When a new dispensation of the Gospel opens, does authority need to be conferred by angels (or the Lord) in the new dispensation? Without a commission from Christ, could Nephi continue his ministry into the new dispensation? Why not? Did the end of the prior dispensation of carnal commandments require a new delivery of power to those serving into this era of a new covenant? (Hebrews 8: 13.)
Does the Lord's reference to "when [He is] again ascended into heaven" reveal anything to Nephi? To us? Does it confirm the Lord's status, power and right? Does it confirm, also, the Lord will be leaving the Nephites again? Does it reestablish what they saw when He first appeared, that He now belongs to heaven? Do we need to keep that in mind as well?