You sound like a Mormon, to be honest,

-- and I mean that in the nicest possible way.
Except the Mormon is actually trying to make a case for polygamy; I'm only making a case for inconsistent (or, at the least, flexible) standards.
2 Sam. 12:7-9 reads: "And Nathan said to David, Thou art the man. Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul; And I gave thee thy master's house, and thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if that had been too little, I would moreover have given unto thee such and such things."
First, for David to have been given the "wives" of Saul would have been extremely difficult since we know: a) Saul had only one wife and one concubine (1 Sam. 14:50); and b) Abner appropriated Saul's concubine for himself (2 Sam. 3:7). So where are all of the wives? Answer: They don't exist. There is no documentation that indicates: a) Saul had all these so-called "wives"; or b) David took any wives from Saul.
And yet, we have the words of God himself (or is it Nathan?) saying not "wife" but "wives." If they don't exist, I'd expect an omniscient being to know that, and an inerrant being to tell it like it is.
Second, in the East it was the general custom for a king's successor to take everything possessed by that king. His country, his throne, his harem, his treasures -- his everything!
But we don't see "I gave you his houses" and we do see "I gave you his wives."
Enumerating such things in a formulaic way indicated a complete turnover of power and authority. It was customary to list all kinds of possessions even when such possessions might not have truly existed (e.g., "keeper of the cattle on a thousand hills," "holder of the moon and the stars," "master of your enemies chariots"). In other words, it's a kind of pronouncement. What we have here is God saying to David, basically, "What is your problem? I gave you everything there was to give. I gave you Saul's total kingdom. Nothing was held back from you, yet you still behave like this."
Okay, assuming the Almighty felt the need to "make his case" to David, I still see no reason to assume he'd be constrained by the "forumulaic way" that mere men might express themselves, to the point that he'd say something that simply wasn't true. Again, he says "the house of Israel and Judah" without adding the moon and the stars or even the pyramids of Egypt. What's the justification for considering "wives" to be formulaic hyperbole when nothing else in the statement seems to be anything but literal?
Third, even if only for the sake of discussion we were to say that God DID give to David some as yet unknown/undiscovered wives of Saul -- then such "wives" would merely constitute an aspect of Saul's kingdom that would have naturally had to go to David in that social/cultural era. This statement under those circumstances, again, would still be an example of God communicating a well-known Eastern tradition involving the transfer of kingly authority.
Once again, this assumes that the customs of men would supersede the will of God. He sent angels to spirit Lot's family out of harm's way before raining fire and destruction on Sodom and Gomorrah, so surely he could have arranged for Saul's (hypothetical) wives to slip away if he hadn't wanted David to have them.
So this remark by God, however you slice it, actually falls terribly short of any stamp of approval of polygamy (and is not a true contradiction: A = Non-A).
When God is complaining about how Solomon has fallen short of God's expectations, he doesn't complain about the NUMBER of Solomon's wives, he complains about the CULTURE of Solomon's wives.
I can grant that it may not be a true contradiction -- maybe the ideal is "one man/one woman" but other unions (one man/many women) are acceptable as well. Jesus suggests his "ideal" is no union at all -- no marriage, no children, no attachment to family (even the elsewhere-honored father and mother), no worldly possessions, just stay celibate and single, relinquish all you own, and follow him. To me, that doesn't imply condemnation of those who fall short of "the ideal".
Remember that humanity has "free will" -- that is a fundamental aspect of Christianity. And within a world where God allows "free will" to exist, he usually works within the framework of the world that has been created by humanity -- and within the world are behaviors that go against God's perfect will. That'd be sin.
Sorry, but "free will" discussions bore the hell out of me. I don't know how Saul had the "free will" not to be unanointed when David was anointed king over Israel, but I don't really want to argue the point. I believe men have free will because there's no God meddling in their decisions - hardening their hearts and what-not. That's another discussion, and I always avoid it, whether with theists or "material determinists" or whatever they call themselves.
To address your point, working within the framework of the world wouldn't prevent God from making his wishes known. He wasn't shy about saying don't eat shellfish, or don't wear mixed-fiber garments, straight out. Is there someplace in the Bible where he's equally plainspoken on the subject of polygamy? Clearly, most men, even in the time of David and Solomon, lacked the wealth to support multiple wives. The New Testament formula of "a man" and "a woman" could have been merely a general statement, applicable to most unions, but expressing no implicit disapproval of exceptions of one sort or another.