"The sealing of earthly covenants and performances is conditional and depends upon the recipient's personal commitment and worthiness. If a person who has received the Holy Spirit of Promise subsequently becomes unrighteous, the seal is broken until full repentance and forgiveness occur" (DS 1:55; 2:94-99).

"The necessity of sealing by the Holy Ghost is emphasized in the following passage: "All covenants, contracts, bonds, obligations, oaths, vows, performances, connections, associations, or expectations, that are not made and entered into and sealed by the Holy Spirit of promise,…are of no efficacy, virtue, or force in and after the resurrection from the dead; for all contracts that are not made unto this end have an end when men are dead" (D&C 132:7). Earthly representatives of the Lord, such as bishops and elders may be deceived by an unworthy person, but no one can deceive the Holy Spirit, who will not ratify an ordinance received unworthily. This safeguard is attached to all blessings and covenants associated with the gospel of Jesus Christ.
The ultimate manifestation of the Holy Spirit of Promise is in connection with having one's calling and election made sure—that is, receiving "the more sure word of prophecy" testifying that an individual is sealed up to eternal life (D&C 131:5). The Holy Spirit of Promise validates this blessing or seals it upon the person. Referring to the Holy Spirit of Promise the Lord says, "This Comforter is the promise which I give unto you of eternal life, even the glory of the Celestial Kingdom" (D&C 88:4; cf. MD, pp. 361-62)".

After a person has advanced in righteousness, light, and truth to the point that the fulness of the ordinances of the house of the Lord have been received so that he has been sealed up unto eternal life, and his calling and election has been made sure, then as expressed in the Prophet's language, the law is: 'If a man commit adultery, he cannot receive the celestial kingdom of God. Even if he is saved in any kingdom, it cannot be the celestial kingdom.' (History of the Church, vol. 6, p. 81; Doctrines of Salvation, vol. 2, pp. 92-94.)" (Mormon Doctrine, p. 23)

JOEL - The "Holy Spirit of Promise" is the power by which all ordinances and other righteous acts performed on this earth, such as baptism and eternal marriage, are ratified, validated, and sealed in heaven as well as on earth. The "calling and election made sure" is just one of those ordinances that is sealed by the Holy Spirit of promise.
In the History of the Church reference McConkie used in that last paragraph, Joseph Smith seemed to just be making an off-hand remark to the Apostles about how one who commits adultery cannot obtain the celestial kingdom. That is true if the person does not repent. Joseph was probably not specifically talking about those whose calling and election were sure, although McConkie used his remark in making his point about no forgivness for the sin for such people.
Here is some additional information from McConkie:

"And as to the fact that the sealing power cannot seal a man up unto eternal life if he thereafter commits adultery, the Prophet says: "if a man commit adultery, he cannot receive the celestial kingdom of God. Even if he is saved in any kingdom, it cannot be the celestial kingdom." (History of the Church, vol. 6, p. 81.) Thus, even though a man's calling and election has been sure if he then commits adultery, all of the promises are of no effect, and he goes to a telestial kingdom, because when he was sealed up unto eternal life, it was with a reservation. The sealing was not to apply in the case of subsequent adultery. In other cases, through repentance, there is forgiveness for this sin which is second only to murder in the category of personal sins. (I Cor. 6:9-11; 3 Ne. 30; D. & C. 42:24-26.) (Doctrinal New Testament Commentary, Vol. 3, pp. 344, 346-347)

Joseph Fielding Smith said:
"In the Doctrine and Covenants 42:24-26, the Lord has given us a key to this situation(adultery). If a person commits adultery and then repents with all his heart, he may be forgiven. If he repeats the offense, he is not to be forgiven, but is to be cast out. As I read it, the Lord has not provided that, under those circumstances, he can come back again.
Now this revelation was given before the endowment was made known. Since that time when a man is married in the temple, he takes on a solemn covenant before God, angels, and witnesses that he will keep the law of chastity. The if he violates that covenant it is not easy to receive forgiveness". (Doctrines of Salvation, vol. 2, pp. 93)
and then he goes on to give the same Church Histroy quote from Joseph Smith.

So after reading the entire references you gave in the first paragraph(DS 1:55; 2:94-99), Joseph Fielding Smith was talking generally about people who have received the common church ordinances(baptism, temple marriage etc.) that are sealed by the Holy Spirit of promise. For these people forgiveness for adultery is possible but not an easy thing, and not possible if the offense is repeated.
In the last paragraph Bruce R. McConkie was talking specifically about those whose calling and election have been made sure. For these there can be no forgiveness(Mormon Doctrine, p. 23).
Bruce R. McConkie also said about adultery: "Forgivness with with resultant celestial salvation depends upon the light and knowledge of the one guilty for the grossly wicked adulterous relationship" (Mormon Doctrine, p. 24).
Joseph Smith said:
"According to the Scriptures, if men have received the good word of God, and tasted of the powers of the world to come, if they shall fall away, it is impossible to renew them again, seeing they have crucified the Son of God afresh, and put him to open shame; so there is a possibility of falling away: you could not be renewed again, and the power of Elijah cannot seal against this sin, for this is a reserve made in the seals and power of the Priesthood." (Teachings, p. 339)

And in Hebrews it says:
"For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
And have tasted the good word of God, and the powers of the world to come,
If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame." (Hebrews 6:4-6)

And Jesus said:
"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. (Matt. 12: 31)

So to sum it up, I believe what all this means is that if someone commits the sin of adultery, who has had their calling and election made sure, sealed by the Holy Spirit of promise; it is as if they are denying the Holy Ghost, a sin for which there is no forgiveness, which would disqualify them for the Celestial kingdom.
For all other church members who have only received the regular ordinances of the church, forgiveness for adultery is possible but not easy. And forgiveness in the sense that they can obtain the Celestial Kingdom is not possible, if they have committed adultery once, repented for it and commit it again (Doctrine and Covenants 42:24-26). They will receive a lower Kingdom of glory.