This just sounds like your personal interpretation of things more than anything.
So far all evidence points to the fact that it is an ironclad rule regardless of the exact reasoning behind it (the things you talk about, like "getting the companies to cooperate" or whatever are just reasons for the rule itself being in place, and don't really do anything to contradict it). No character has broken this rule. Until a character gets in that doesn't follow this rule, there is no reason to believe it isn't a rule. If and when we get a character in Smash that has never truly appeared on a Nintendo system, like Master Chief, Marcus Fenix, Kratos or Blinx the Time Sweeper, then and ONLY then I'll believe you. Until that day however, you are wrong.
That's not how evidence works in cases like this. To prove it's ironclad, you need to prove that Sakurai will never in any way possibly add someone like Master Chief. But that's kind of impossible as his statements prove completely otherwise.
Now I did very slight misinterpret what the statement meant about "as a courtesy". I am still right that it's not an ironclad rule, because it doesn't even remotely make sense that way. But it means that he has a higher chance of adding a character with an appearance. He takes the appearance into consideration. It does not determine everything, though. Which you seem to be under the impression. Despite him not once implying or suggesting it's the only thing that matters. Obviously, yes, to what we can see with the patterns, and all they legitimately could prove at best, a Nintendo-appeared character is more likely than one without an appearance. But that's as far as it goes.
If you want an ironclad rule, it's 3rd party characters must originate in a video game first. 1st and 2nd party characters can be considered iffy, as some appeared outside of a video game as promotional materials. Most likely, those are clear exceptions to the rule even then. Which makes a lot of sense anyway. Another one is they must at least have a video game appearance to be remotely possible. Also, the ballot implies game origin is way more important than anything else to be eligible.
You haven't even given proper evidence either. You gave a pattern of why it's hard to believe anyone without a Nintendo appearance has a chance. That's more than a reasonable stance to take, but it's just an opinion. His statements contradict the idea of it being a hard rule. In face, his statements show that he's iffy on the situation, with the best and most accurate statement being a "Maybe". It's kind of impossible to have a hard rule when the answer is pretty much Maybe anyway.
It's a hard fact that it's not in any way a hard rule. Being he literally talked about Cloud and said he wasn't sure if he would've added him. That already proves the fact that it isn't the sole determiner. If it was a hard rule, he would've said "No I would not have added him." The evidence and his statements very clear show it's just something he needs to look at as a possible point to their inclusion.
I could put it in many other ways, but overall, ironclad rules have absolutely zero room for alternate analysis or "maybes". It's a hard yes or no. That's what ironclad means. It's pretty blatant that it was not ever ironclad(now, that could change with another director, sure. But this is Sakurai, and he pretty clearly has not made it a hard rule either. Heck, most of his so-called hard rules are just guidelines, with barely any hard rules to begin with. That's not he thinks as a developer. He likes to keep an open mind. Here's a hard rule for you though that actually is true, and the reason is because he made it clear; A character cannot share a costume with another while having different moves. They need to be separated if they have even the remote difference). Paraphrasing, but that's exactly what it means and why even the remote difference required a separation. This is why Dark Pit, Dr. Mario, and Lucina will never be costumes as long as Sakurai is directing. He believes a character's moveset is what makes them a separate playable character, no matter how minute the difference. At best, they can share the same slot, but not the same "body". Mii Fighters are an example of this, being basically akin to how you could select Pokemon Trainer in Brawl.
