In order to understand any law you must first understand it's original intent, otherwise you my follow the exact letter of the law and yet still be braking the law.
In my opinion, the original intent of many biblical laws (that seems unusual to us today) was to keep followers from worshiping pagan gods and/or from following their common rituals or practices. This (again in my opinion) is the root of many of the laws we don't really understand today such as the rounded haircut and wool and linen interweaving prohibitions.
But the thing is.... nothing is static. Things change. New customs and rituals arise, and old customs and rituals fall out of favor.
So to answer your question. Yes, I believe that at the time those laws were made it was a sin to do those things, but not for the reason YOU think. But because of the original intent, which was to keep followers of God from following the ways of the pagans.
Fast forward to today.
Are there any pagan cults that go around wearing special rounded hair cuts and specially woven garments of wool and linen to distinguish themselves? no? Then it is no longer prohibited. Get it?
On the other hand... are they MANY MANY practices of pagans and atheists today that follows of God should not immolate that are NOT mentioned in the bible? Absolutely.
We are called by God to be different and to live our lives differently than the Pagans. So if pagans and atheists in this day in age love to get their face and tongue pierced, or cover themselves in tattoos to distinguish themselves from others. Then it is a sin to emulate them.
Followers of God should not worship pagan Gods or emulate how they look or follow their rituals.
It's as simple as that.
Now that I have addressed all the seemingly weird commandments, I will once again try to address the slavery issue which you seem to just not get (or are purposely pretending not to understand)
For simplicity sake... lets look at a modern day issue.
Let say for instance.... that you personally are against the death penalty is all instances. You think it's abhorrent and barbaric and that it should never be imposed by a civilized people on anyone, and that life in prison is a far better better punishment anyway, ect.
Now... even though you think the above... you are in the minority and capital punishment is legal.
So.... do you just stand by and do nothing if some state decides to say... bring back being drawn and quartered as the method of execution?
or..
do you say... um... you know what... I don't even believe in capital punishment... BUT, if you are dead set on carrying out such a barbaric thing, then I believe it should be done in a more humane way, like by lethal injection.
... see where I am going?
Does your advocation of using lethal injection in executions mean you approve of capital punishment?
of course not.
And in the same way, God laying out rules for how masters should treat slaves in a humane way in no way means that God approves of slavery.
I hope this clears things up for you.
First most of what you have offered is opinion and as such is worth no more of less than anyone else's. Hard facts are what I asked for.
Second Frequently when I talk to protestants about a theological concept they counter with "That was the old covenant and is no longer valid through Jesus and the new covenant." We clearly see this practiced with the lifting of the ban on certain foods.
Third, You sad that some of the prohibitions were so that we would not emulate pagan practices. If you study middle eastern customs you will see that Circumcision (Both male and female) were practiced before the Covenant with Abraham and are still practiced by many pagan groups. So much for not emulating pagan practices
Fourth you are forgetting the Catholic Church. It was the Catholic Church that lifted the mandate requiring circumcision at the Council of Jerusalem. It was also the Catholic Church that changed the day of worship from Saturday to Sunday. Both of these were defined in the OT as "perpetual ordinances". Perpetual means for ever.
So there you have it. The prohibition against tattoos was Old covenant, it is no longer prohibited by the Catholic Church, and pagan practices have no bearing on the topic.
Feel free to have the last word.