Lots of conjecture and interpretation here.
You're right about that. All historical hypotheses involve interpretation and conjecture. Making a superficial interpretation that neglects 95% of the evidence is still interpretation and conjecture...it's just a very uneducated interpretation based on a very faulty basis.
There is at least one serious alternative interpretation....that God did want them to speak more forcibly about slavery, and it was the writers of the New Testament themselves who weren't ready to write that yet. Thus, the guiding principles of life which they had learned from God were there, and you can still see Paul carrying those principles to their natural conclusion in telling Philemon to free his slave, but a specific command against slavery wasn't there because slavery was just such a basic assumption about societal life in the 1st century that the New Testament authors never understood they should totally condemn it. I'm not ruling that interpretation out.
Of course, the dumbest possible interpretation would come from taking a single line out-of-context, ignoring the relevant history, throwing in a modern context that had absolutely nothing to do with the situation at hand, and then pretending that you've said anything at all about the fact. When you do that, you're interpreting all right, by what you've chosen to include and not include and by what context you've chosen to place it in. However, an interpretation based on such a miniscule selection of the evidence says far more about what's going on in the interpreter's head than what was actually written.
But if God is allowing this harsh abuse for the sake of appearing attainable to those who are inflicting that mistreatment on those that serve them... thats not a God I would ever serve. Either God is for justice or He isnt. Damn the grey areas.
Love your neighbor as yourself
Do to others what you would want them to do to you
Consider others more than yourself
He who wishes to be great must become a servant, for those who put themselves first will be last in the Kingdom of God
The Son of Man came to Earth to serve, not to be served
If you are my disciples, then choose to serve rather than to be served
There is no slave or free but we are all one in Christ Jesus
Slaves and slaveowners are brothers
No one could actually follow Jesus's commands or Paul's interpretation of them and remain a slave-owner.
God is for justice. The question is how to make the justice actually happen. As Bernie said recently, all the screaming in the world doesn't accomplish shyt.
Slavery, even the form of temporary "debt bondage" being practiced in Israel at that time, is not a grey area.
However, actually getting slavery ended, like actually getting women equal treatment, was something that was going to take time in that society. In the 1st century, every single society in existence just assumed that slavery was part of life. You could try to combat that directly by introducing an anti-slavery claim that automatically pits people against each other, that is automatically rejected by all in power, and that doesn't do shyt to change the status quo, but you're proud of that system because you "said what needed to be said". Or you can create a system that actually converts both the powerful and the weak to justice and righteousness, and thus transforms the system from the inside. The principles laid out in the New Testament clearly make slave-owning completely untenable. And the letter to Philemon shows that working in practice. The fact that slaveowning is never fobidden outright is only a matter of technique in attaining justice, not a difference in the view of justice.
As I pointed out already, Christian bishops were the first in history to call for the universal abolition of slavery, within just a couple centuries of the founding of the church. You want to go ahead and name the next system of justice to call for the abolition of slavery? What's your own system of justice, and how does it rule slavery out without relying on Christian foundations of selfhood, brotherhood, and morality?
I'm not sure what you mean by "allowing this harsh abuse". Do you think God should step in and supernaturally intervene any time an injustice occurs? That can be a long and meaningless discussion, because it's obviously not how the world works. God seems to have created the world with true free will that allows humans to choose to be good to others or choose to be evil to others. Humans are the ones who allowed slavery, as long as they allowed it. And if slavery was going to be stopped by humans, it was only going to happen if the people in power stopped it by being transformed from within, not because some powerless little sect of Judiasm wrote a book that told everyone it was wrong.