Does the Mormon doctrine of exaltation implicitly require acknowledging the existence of sentient extra-terrestrial life?

Before we dive into the answers, we need to understand the Mormon perspective on the doctrine of exaltation.

The focus of all LDS teaching is on salvation through Christ and His Atonement. Exaltation is all about bringing us back to Christ and to know Him by becoming like Him. Exaltation is not "getting your own planet." That phrase is foreign to Mormons. The Gospel Principles manual says:

Exaltation is eternal life, the kind of life God lives. He lives in great glory. He is perfect. He possesses all knowledge and all wisdom. He is the Father of spirit children. He is a creator. We can become like our Heavenly Father. This is exaltation.

And with exaltation come these things as some blessings:

Our Heavenly Father is perfect, and He glories in the fact that it is possible for His children to become like Him. His work and glory is “to bring to pass the immortality and eternal life of man” (Moses 1:39).

Those who receive exaltation in the celestial kingdom through faith in Jesus Christ will receive special blessings. The Lord has promised, “All things are theirs” (D&C 76:59). These are some of the blessings given to exalted people:

1. They will live eternally in the presence of Heavenly Father and Jesus Christ (see D&C 76:62).
2. They will become gods (see D&C 132:20–23).
3. They will be united eternally with their righteous family members and will be able to have eternal increase.
4. They will receive a fulness of joy.
5. They will have everything that our Heavenly Father and Jesus Christ have—all power, glory, dominion, and knowledge (see D&C 132:19–20). President Joseph Fielding Smith wrote: “The Father has promised through the Son that all that he has shall be given to those who are obedient to His commandments. They shall increase in knowledge, wisdom, and power, going from grace to grace, until the fulness of the perfect day shall burst upon them” (Doctrines of Salvation, comp. Bruce R. McConkie, 3 vols. [1954–56], 2:36; italics in original).

Exaltation is a broad and deep topic of LDS doctrine, much of which can only be learned, as the LDS believe, by revelation and attending the temple and receiving those ordinances. It is sacred to them.

With that in mind, remember that the teaching of any speculation or topic not directly relevant to our salvation is discouraged in the LDS church. You know how it can be: opinions or ideas get taught in class or over the pulpit, and maybe get misconstrued as truth, then all of a sudden you have congregations teaching and absorbing false doctrine, and their focus is removed from the Atonement of Christ.

So as far as I know, there is no official doctrine in the LDS church at this time which directly answers your third question. The LDS would probably say, "We don't know. It hasn't been revealed." Any other attempted answer would merely be speculation based on reasoning and maybe science, but not on the revelations from God which the LDS would require in order to account it as doctrine.

But your first two questions have more secure answers:

1. In the same chapter in that manual referenced above, it says

To be exalted, we first must place our faith in Jesus Christ and then endure in that faith to the end of our lives. Our faith in Him must be such that we repent of our sins and obey His commandments.

He commands us all to receive certain ordinances:

1. We must be baptized.
2. We must receive the laying on of hands to be confirmed a member of the Church of Jesus Christ and to receive the gift of the Holy Ghost.
3. Brethren must receive the Melchizedek Priesthood and magnify their callings in the priesthood.
4. We must receive the temple endowment.
5. We must be married for eternity, either in this life or in the next.

In addition to receiving the required ordinances, the Lord commands all of us to:

1. Love God and our neighbors.
2. Keep the commandments.
3. Repent of our wrongdoings.
4. Search out our kindred dead and receive the saving ordinances of the gospel for them.
5. Attend our Church meetings as regularly as possible so we can renew our baptismal covenants by partaking of the sacrament.
6. Love our family members and strengthen them in the ways of the Lord.
7. Have family and individual prayers every day.
8. Teach the gospel to others by word and example.
9. Study the scriptures.
10. Listen to and obey the inspired words of the prophets of the Lord.

Finally, each of us needs to receive the Holy Ghost and learn to follow His direction in our individual lives.

2. From the same source as above, there are some scriptures referenced:

• D&C 132:3–4, 16–26, 37 (pertaining to exaltation)
• D&C 131:1–4 (eternal marriage is key to exaltation)
• D&C 76:59–70 (blessings of celestial glory explained)
• D&C 84:20–21 (the power of godliness is manifest through priesthood ordinances)

I was going to quote them, but it got kind of long. You can read them here.

There are also more scriptures to be found in the Bible and other books. A satisfying, but perhaps not comprehensive list, can be found in the Topical Guide under "Exaltation."

I would not expect this answer to satisfy some of the readers. Mason Wheeler has already explained, in the comments, the reason for this, and I will quote it here for emphasis, because I would suggest the same in this case:

It's worth noting that a good, concise explanation of Exaltation is very difficult to give, as it's basically the highest point of LDS doctrine, and to understand it well--beyond a brief overview that's more of a caricature than anything actually useful--requires an understanding of a lot of other doctrine first. You would get a better explanation by speaking with LDS missionaries, (who do explain Exaltation, after building up to it in a well-structured manner,) than by looking for specific answers here without laying the groundwork. "Line upon line, precept upon precept." (Isaiah 28:10)

(On a personal note, I often do this type of thing in my studies. For example, if I wanted to learn calculus but only knew arithmetic, I wouldn't go check out a calculus book and expect to understand it. I visit with professors and practice the problems on my own until I can grasp the more advanced concepts.)