There is only one standard, the one God laid out. He didn't lay out a standard foe concubines. He didn't lay out a standard for inheritance. As far as I can tell a marriage contract isn't even scriptural. It certainly isn't commanded or even mentioned that I can tell.
This is the great danger in filling in the supposed blanks. We find out we're just coloring outside the lines. Abraham slept with his sister and then tried to pawn her off on a couple of his buddies to save his own hide from an imagined threat. Just because a righteous man did something doesn't mean you should imitate it or that it was God's will. It just means it happened.
What you're doing is assuming there are no blanks or if there are, they can't be filled. But it could alternatively be true that blanks were left by God to us to fill in. Or that they weren't blanks, but commonly understood cultural practices that weren't necessary to expound upon. IOW, you're making scripture to be more all encompassing than it actually is.
The scriptures are law, but they don't give detailed definitions like modern law. They were more like protections added to common practice than all encompassing definition.
For example, the Bible referenced dowry's for wives. But what is the dowry for? Who gets it? How much should it be and who decides how much? All those are blanks left open in marriage. If you want a biblical marriage to a wife you HAVE to fill in the blanks. No way around that. And the presence of dowry potentially implies a contract and points to the fact that the law is buttressing/correcting the cultural practice of the day.
If you want to understand how the Ancient Hebrews did dowry, you'll have to look at contemporary secular law; which as it so happens also explains concubinage to an extent.
There is no explanation of concubinage made in scripture. We can't add one. No matter how you slice it, in God's eyes your concubine will just be a wife that you treat differently. And that's the real question to be asking here, not whether or not we can be having sex with some woman without having a permanent responsibility to God for her (which is what is at the heart of a lot of the concubine questions) or find a work around to marry an ineligible woman. How does God differentiate between these women? How is the relationship started and ended any differently than a marriage?
Where is it said we can't add a definition if it's not given? And I'm not even trying to 'add' one, but rather understand from the historical record what they meant when they used the word concubine. Thats not 'adding', its basic reading comprehension.
Nor am I looking for an out to have sex without responsibility. Hebrew concubines weren't mistresses, they were kept in house and cared for; generally for life. Though Abraham does bring up an argument for them not being permanent I couldn't say either way yet for sure.
You have to remember that there isn't even a word for wife or marriage so that they can be differentiated from a concubine. There are simply women who are mastered. That's all the language tells us. So if there is no such thing as a wife how can we differentiate her from a concubine? And if there's no difference then who cares?
That is not consistent with ancient practice nor the language. Concubines were a different thing. We don't get to lump them in with wives just because they also happen to be women. The common practice of marriage was just what women did generally; no name was needed for the default. But when a different alternative came along it got a new name to differentiate it from usual practice. That doesn't mean the standards of the common practice apply to the alternative. They might; but it could also strongly imply they don't.