1) The divorce law (a divorced woman should not remarry):
To the married I give charge, not I but the Lord, that the wife should not separate from her husband [7:10]
(but if she does, let her remain single or else be reconciled to her husband)--and that the husband should not divorce his wife. [7:11]
And he said to them, "Whoever divorces his wife and marries another, commits adultery against her; [10:11]
and if she divorces her husband and marries another, she commits adultery." [10:12]
2) Jesus' coming in clouds to gather his elects:
For this we say unto you by the word of the Lord ... [4:15]
For the Lord himself will descend from heaven with a cry of command, with the archangel's call, and with the sound of the trumpet of God. And the dead in Christ will rise first; [4:16]
then we who are alive, who are left, shall be caught up together with them in clouds to meet the Lord in the air; and so we shall always be with the Lord. [4:17]
And then they will see the Son of man coming in clouds with great power and glory.[13:26]
And then he will send out the angels, and gather his elect from the four winds, from the ends of the earth to the ends of heaven. [13:27]
Note: gMark "in clouds" ('ἐν νεφέλαις') is the perfect match of "in clouds" in 1 Thessalonians 4:17. However, in the corresponding passages, gMatthew has "on the clouds of heaven" (26:64) (similar to "with the clouds of heaven" in Daniel 7:13) and gLuke has "in a cloud" (21:27).
Out of the four "revelations" in the Pauline epistles, two of them reappear, albeit considerably rewritten, in gMark (dated 70-71). That's remarkable and most likely not the product of chance.
What about a third one, the Last Supper:
I understand now the Last Supper in 1 Corinthians
(11:23-28) is an interpolation. I explained
that here. However, I think "Mark" might
have got ideas about the Last Supper ritual
from these verses in 1 Corinthians:
"I speak as to wise men; judge ye what I say. [10:15]
The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? [10:16]
For we being many are one bread, and one body: for we are all partakers of that one bread. [10:17]
Behold Israel after the flesh: are not they which eat of the sacrifices partakers of the altar?" [10:18]
Note: how could Paul present verses 16-18 (more so 16 & 18) as a product of his thoughts, if he knew about Christ's Last Supper?
Also, in the 1 Corinthians letter, the "Last Supper" setting as a Passover meal might have been inadvertently inspired by Paul:
1Co5:6-8 "Your boasting is not good. Don't you know that a little yeast works through the whole batch of dough? Get rid of the old yeast that you may be a new batch without yeast--as you really are [if there was no boasting!]. For Christ, our Passover, has been sacrificed. Therefore let us keep the Festival, not with the old yeast, the yeast of malice and wickedness, but with bread without yeast, the bread of sincerity and truth."
Paul's imagery relates to the Passover (one particular day) and the overlapping Festival of the unleavened bread (14 days), when bread without yeast (a symbol of purity "sincerity and truth") is eaten, instead of the leavened bread (symbol of impurity "malice and wickedness"). Naturally, Christ's sacrifice is associated with Passover (the day in the year) as a turning point: before, "malice and wickedness"; then and after "sincerity and truth".
What about the fourth one? this one (2 Corinthians 12:1-9a) is about Paul's ministry and could not be fitted into the timeline of the gospel.
It seems "Mark" felt obligated to include all of Paul's two items (command & words) about Jesus (appearing in the epistles) into the "historical" setting of his gospel as, allegedly, heard by eyewitnesses.
If it is so, then "Mark" knowing about some of Paul's letters is the only plausible solution.
PS: 1 Corinthians 2:14 "But the natural man does not receive the things of the Spirit of God, for they are foolishness to him ..." (after Peter got followers in Corinth)
Let's compare that with:
Mark 8:33 "You [Peter] do not have in mind the things of God, but the things of men."
1 Thessalonians 5:13 "... be at peace among yourselves."
Let's compare that with:
Mark 9:50 "... be at peace with one another."