You're mixing a bunch of facts together that have nothing to do with the original point. It feels like we're talking past each other, and getting hung up on "know each other" so I'll go back to the only relevant point ; Cat and Ned arranging to have Robb marry Margaery.
Your first point was basically "why didn't Ned and Cat marry Robb to Margaery?"
And the answer to that is Cat and Ned knew NOTHING of Margaery's personality. They don't know the Tyrells like that. That is what I mean by "knowing" the other great houses. Yes they know the sigils of the other houses, they know their house words, they know their castle, and the region they rule, highborn kids learn that shyt in their studies, and that is what I mean by "know of" the other houses.
But there is a difference between knowing a house's words, and knowing their children on a personal level. The Starks have regular feasts with other nothern lords, people from the mountains, and because Cat is from the riverlands, people from that region as well. So Ned and Cat personally know the children of those lords, have met them, talked with them, seen what their personality is like - all things they do not have with the Tyrells, or the majority of every house south of them.
You are acting like they have a dating profile of every girl born from a major house, and Margaery's profile read "Pretty, long hair, cute dimples, smart and funny, loves long walks on the beach, is a bad bytch".
They didn't know what she looked like. They didn't know if she was smart. They didn't know if she was cunning. They didn't know if she was friendly. They didn't know if she was anything of a good match for him. All they would have known, and by record, not by any discussion or incentive to care, was that Mace had a few heirs, a boy and a girl (tv show).
That's it.
Nothing more.
So asking "why didn't Ned and Cat marry her to him" makes no sense. During peace times, they wouldn't have given a fukk about a Tyrell daughter, they've never married into that house before, they don't trade with them (the Starks trading is done with White Harbor, plus their grow their own crops in their glass garden), they haven't spoken to them damn near two decades, hell the Tyrells didn't even come to Winterfell whenever King Robert visited.
So please tell me the time frame when the thought would have even crossed their mind for Cat to say "Hey Ned. Even though we've never met a Margaery Tyrell, and haven't had any dealings with the Tyrells since we've been married, I think we ought to marry the heir to Winterfell to her - not any of our bannermen, not any of my fathers bannermen, no, just this random Tyrell girl that we know nothing about. She'd be perfect for our boy."
By the time Cat actually got to meet Margaery, the war had started and she was already Renly's queen.
Obviously dude. I've already made note of that. But you can go through the lines and see the tendencies of great houses, and the Starks have never married with the Tyrells. As homie above me said, the Starks are mostly isolationist. They haven't married Tyrells. They haven't married Lannisters. They haven't married Martells. They haven't married Targaryens.
They sometimes marry from the vale or riverlands, but it's mostly other northern houses. I mean, Ned's parents were cousins.
You listing a bunch of southern marriages, when I'm talking about a southern and far north marriage doesn't do anything to refute the actual point I'm making. And it doesn't give any insight into why it would make sense for Cat and Ned to arrange for Robb to marry that particular house peace times, which is the context of this conversation.
This was orchestrated by the Tyrells during war times as a power grab while Sansa was a hostage in King's Landing. That's not the same as Ned and Cat marrying their son to a Tyrell during peace times, the context of this discussion.
Ned met her at a tourney, and as a second born, was not going to get arranged to marry anyone like his brother was. Remember, this topic is about ARRANGED marriages, not if two people meet each other at a tourney and fall in love. That would only be used as an example if Margery and Robb had met at a tourney, danced, and fell in love. We know that isn't true because Robb had never been out of the north before calling his banners to march south.
Robert didn't grow up in the Stormlands. He grew up alongside Ned in the Vale, met with Rickard Stark, and fell in love with his daughter after meeting her. Again, context nothing like northern parents arraigning for their son to marry a girl they don't know from a house they have never married before.
That's because he wanted Ned to be his hand, and needed good incentive to bring the man and his family to King's Landing. Come on man, none of these examples fit the context at all of what we're talking about; Cat and Ned picking Margaery of all people, during peace times, to marry Robb. And it would have to be during peace times, so somewhere between Robb being 13 and 15, because no other timeframe makes sense. Margaery was already married off by the time Ned got imprisoned. And by the time Renly was killed, Robb was already engaged to a Frey girl. So when would they have arranged to have those two marry?