Do Mormons actually believe in any sort of supreme being/ultimate reality/"Absolute"?

If you are ever in the presence of God the Father, you may be privileged to ask Him for yourself whether He has a Father, or perhaps the question will have already be answered for you at that point. But to us there is one God, and He is our Father. A cousin, niece, nephew, or even grandfather does not and cannot take the place of a father.

Fatherhood, and particularly Divine Fatherhood, is supreme. How many fathers do you have? Which of the above relatives do the Scriptures require that you honor? (In Moses' day, people were stoned for dishonoring their parents. It was a sin punishable by death.) God's feelings on the subject have not changed. "Thou shalt have no other gods before me; For I, the LORD your God, am a jealous God" (Exodus 20:3-5). You will never find a faithful latter-day saint worshiping Joseph Smith or a purported heavenly uncle or exalted sibling or anyone else but the true God Himself, that God who is the Father of our spirits. Father is a most meaningful and rich term, however, and is rewarding to those who are the children. Towards those who are not children, it is a more distant or perhaps even an alien relationship. In that sense it is a relative one. But since we know who our Father is, it is absolute and unchangeable. We will never change our minds about this. We love our Father forever, because He first loved us, and is our Father in very deed. We may have the testimony that Joseph Smith will receive his exaltation, just as Abraham, Jacob and Joseph did anciently, but that is a matter unto himself and his eternal family. It will do us no good if we do not enter into those covenants and receive those ordinances from the God of Heaven for ourselves and our family, and rise to "the measure of the stature of the fulness of Christ" as the Scripture says we may. That will be a consummate gift from Father to child, and the eternal continuation of the Family Business, the true rite of godhood as a literal heir with Christ (of which the Scriptures also testify), that is, of having limitless children, saving and exalting them according to their love and obedience.

We owe no adoration towards those who are not our God, just as we owe no honor toward those who are not our earthly parents. We have one God. To us there is only one God. This makes perfect sense once we abide by His covenants pertaining to our earthly family. Only to those who honor their father and mother can extended family relationships be meaningful or even exist at all. In a similar manner, our more distant and less proximal relationships may be discoverable, but only after we have obeyed God, who is the Father of us all. Otherwise, we will be spiritual orphans, miserable and alone, and we will be without kin near or far to comfort us. This is the final state of those who reject God. The Scriptures are clear: Those who forsake God the Father for their brother or sister or children are not worthy of Him, while those who receive the Father will receive a full family, many times more than those who forsook them because of their loyalty to God. God gave each of us a family, and He can do it again. Nonetheless, what God hath joined together, let no man put asunder. Our duty is to love God first by keeping His commandments, and our neighbor secondly as ourselves.

In all the universe, "Father" is a unique role, and together with "Mother", is the most fulfilling and supreme of all powers, positions, or trusts.

With regards to the OP's number 3 point:

The Mormon God doesn't act as the ground and source of all existence...

I refer you to D&C 88, which proceeds as follows:

the light of Christ...which giveth you light...[and] quickeneth your understandings...proceedeth forth from the presence of God to fill the immensity of space...The light which is in all things, which giveth life to all things, which is the law by which all things are governed, even the power of God who sitteth upon his throne, who is in the bosom of eternity, who is in the midst of all things.

Here the light of Christ is equated with the power of God, and in adjacent verses is stated to be the power by which the sun, moon, stars, and earth were made and give light. This seems fairly analogous to classical theistic views of God and His creative energies exhibiting a sustaining power, though perhaps somewhat softened, since it is not reality or existence but rather life and order that are sustained.

I want to address points 2 and 3 in your question, as a supplement to depperm's answers.

Point 2

God the Father is the supreme figure of Mormon theology. All of us, including Jesus, are God the Father's spirit children. This relationship will not change. Even if and when we are exalted, God the Father will still be our Father. A recent essay published on the LDS Church's website entitled "Becoming Like God" has this paragraph:

For some observers, the doctrine that humans should strive for godliness may evoke images of ancient pantheons with competing deities. Such images are incompatible with Latter-day Saint doctrine. Latter-day Saints believe that God’s children will always worship Him. Our progression will never change His identity as our Father and our God. Indeed, our exalted, eternal relationship with Him will be part of the “fulness of joy” He desires for us.

Mormons believe that family is the basic organizational unit both on Earth and in heaven and that this will continue through the eternities. This includes our heavenly family, the father of which is God the Father.

Addressing the relationship between God the Father and God the Son (Jesus Christ specifically): throughout the scriptures Jesus always shows deference to God the Father.

John 5:19 (Christ has learned conduct from the Father):

Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.

Luke 22:42 (Christ submits to the Father's will):

Saying, Father, if thou be willing, remove this cup from me: nevertheless not my will, but thine, be done.

1 Cor. 15:28 (Christ is subject to the Father):

And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.

Helaman 5:11 (Book of Mormon, Christ has received power from the Father):

And he [Christ] hath power given unto him from the Father to redeem them from their sins because of repentance.

Doctrine & Covenants 76:107 (Christ will deliver the kingdom to the Father):

When he shall deliver up the kingdom, and present it unto the Father, spotless.

Point 3

One scripture that I think will demonstrate that God the Father in Mormon theology is, as you phrased it, "sustaining all of reality in existence from moment to moment by his creative energies".

Doctrine and Covenants 88 (emphasis mine):

45 The earth rolls upon her wings, and the sun giveth his light by day, and the moon giveth her light by night, and the stars also give their light, as they roll upon their wings in their glory, in the midst of the power of God.

46 Unto what shall I liken these kingdoms, that ye may understand?

47 Behold, all these are kingdoms, and any man who hath seen any or the least of these hath seen God moving in his majesty and power.

Short answer is that the Mormon God, Heavenly Father, is as supreme as it's going to get.

On your misconceptions on the Mormon belief about God:

1. Not creating matter - you are correct in your understanding

You ask the learned doctors why they say the world was made out of nothing, and they will answer, “Doesn’t the Bible say he created the world?” And they infer, from the word create, that it must have been made out of nothing. Now, the word create came from the word baurau, which does not mean to create out of nothing; it means to organize; the same as a man would organize materials and build a ship. Hence we infer that God had materials to organize the world out of chaos—chaotic matter, which is element, and in which dwells all the glory. Element had an existence from the time He had. The pure principles of element are principles which can never be destroyed; they may be organized and re-organized, but not destroyed. They had no beginning and can have no end. (King Follett Sermon)

2. Polytheistic- you are partly correct (Technically more of Henotheism-believing in multiple Gods but only worshipping one) See Also

The First Article of Faith states:

We believe in God, the Eternal Father, and in His Son, Jesus Christ, and in the Holy Ghost.

Latter-day Saints pray to God the Father in the name of Jesus Christ. They acknowledge the Father as the ultimate object of their worship, the Son as Lord and Redeemer, and the Holy Spirit as the messenger and revealer of the Father and the Son. ¹

On becoming like gods ourselves I've written more in this answer (basically to us there is only ever one God 1 Cor 8:5-6).

3. Unclear on subject point. We believe God does have a physical, exalted/perfect body. ² The LDS website says:

He is the ultimate Creator, Ruler, and Preserver of all things. He is perfect, has all power, and knows all things. ... We should seek to know our Father in Heaven. He loves us, and He has given us the precious opportunity to draw near to Him as we pray. Our prayers, offered in humility and sincerity, are heard and answered.

From that it mentions he is the Ruler and Preserver of all things, but it doesn't go into detail about how that is done. The second part mentions that part of what He does is answer man's prayers because He loves us.

Related is Mosiah 4:9

Believe in God; believe that he is, and that he created all things, both in heaven and in earth; believe that he has all wisdom, and all power, both in heaven and in earth; believe that man doth not comprehend all the things which the Lord can comprehend.

₁ https://www.churchofjesuschrist.org/study/manual/gospel-topics/godhead?lang=eng

₂ https://www.lds.org/scriptures/dc-testament/dc/130.22?lang=eng#21

_{All emphasis was added and is not present in the original sources}