I’m going to just focus on the first two items to keep my response manageable.
For both Jesus and Adam, Paul goes beyond the historical man. Jesus Christ is more than a man, and he is referred to in ways that don’t make sense for just a person. Jesus never married; the church is Christ’s bride. Jesus ascended to heaven; Christ lives in us.
So when Paul sums up all of humanity in two figures, they are Adam and Jesus Christ. We are in one or the other – not descended from one or the other. Rather than pointing to Adam being our literal first ancestor, this seems to show that Paul viewed Adam as much more than one man: he is a pattern for all humanity.
Viewing Adam as us would be much easier for Paul than us. Anybody who spoke Hebrew would be hearing the word “humanity” every time Adam’s name was spoken – they’re the same word. For us this seems like a metaphorical addition. For them it was as natural as understanding that Israel was both a nation and a man.
I don’t see how this understanding affects Paul’s theological argument about death. Whether a man or the species, animal death predates Adam. That problem is resolved by looking at what “death” refers to (physical or spiritual, human or creaturely, punctiliar or progressive, a neutral process or a malevolent fiend), not what “Adam” refers to.
Not just “in the beginning,” but “from the beginning of creation” (Mark 10:6)! If creation took six days, which day is the beginning? 
But seriously, look closely at how Jesus doesn’t say “in the beginning.” In both Mark and Matthew 19:4, the preposition is “from” (apo) not “in” (en, as in John 1:1 and the Greek of Genesis 1:1). I don’t think Jesus is talking about some past event. From the beginning – and continuing now – God makes humanity, male and female. This isn’t just about the first couple. This is about all of us. Perhaps Jesus’ insertion of that “from” shows that he reads the creation week as revealing God’s creative activity throughout time, not just the start of it. And if so, he isn’t alone. Psalm 104 and several other passages also restate creation in ongoing terms rather than confined to the first week.
It’s because Jesus sees this as speaking of all of us that he applies it to contemporary life. If we’re just looking for historical detail, why should it be more important that God started a family for Adam by giving him Eve than that God started a family for Jacob by giving him Leah, Rachel, Bilhah and Zilpah? The historical details of Israel’s first parents don’t define Israel the way Adam and Eve define humanity. Because of that, they are relevant to Jesus’ point about divorce. And because the man and woman represent all humanity, and their marriage portrays a universal need for connectedness, they are also relevant to intersex individuals and singles.
The Abel reference is really interesting. First, Cain is not mentioned. Jesus puts his opponents in the place of Cain and all the other murderers: they are the responsible ones. Mentioning Cain would undermine that rhetorical shift. Next, Jesus’ argument depends on Abel’s place in Scripture (at the beginning, in contrast with Zechariah at the end) not his place in history. And the way he calls Abel the first prophet depends on the figure of speech through which Abel’s blood cries out to God. (I went into more detail about this here.)
None of this is a simple recitation of historical events. Jesus is using the details of the story, even the obviously figurative ones, to make his point. The Abel reference is an excellent example of how the New Testament can depend on a literary character as portrayed in Scripture, not the historical person beneath.