My conclusion on the matter is that Christ wasn't necessarily depicting the glory of the second coming in that Moses represented those who died in the faith and would be resurrected to eternal life, and Elijah represented those who would be transalted to eternal life.
Rather, the key to unlock the message that Christ was endeavoring to impart at the time lies in knowing what each one stood for. As you had stated, Moses stood for the Law, and Elijah stood for the Testimony of the Prophets.
It's interesting how both Moses and Elijah were pictured as answering to Christ. Thus the Law and the Testimony of the Prophets must always find agreement with, and meaning in the One who inspired them--God.
Also, because there were three, but then there was One (Jesus), this tells us that the Law and the Prophets find their fulfillment in Christ; and together they disclose the glory of God as they find their oneness in Jesus Christ. (Hint: They are not to be placed in separate tents.)
The fact that God then spoke and told those who witnessed this to listen to Jesus Christ tells us that we should not merely be looking to the Law and the Prophets as an answer to the sin problem, as many in the SDA church often do, but that we should be looking to Christ alone for salvation. For, only in Him will we find eternal life. In fact, Jesus addressed this issue when He had said, "You search the scriptures for in them you think you have eternal life, and these are they which testify of me."
In other words, it's not by works alone, or some intellectual ascent to the truth that we obtain salvation; rather, it's by allowing the glory of Christ's righteousness to absorb our being so that we may become one with Him, thus finding eternal life in His character. The result is that when people look to us, they won't see us; rather, they will see the glory of Christ's righteousness shining as bright as the morning star!
