I'm not really trying to convince anyone of anything.
I'm trying to get into the rules of the game as they're written. While I appreciate the sentiments about asking the DM, I'm not necessarily looking for that type of discussion.
Let me make a more...practical example.
Say a 15th level wizard found a Ring of Three Wishes and got impatient. He decided he wanted a 9th-level spell now and wished it into existence. The DM decides its fine but monkey paws it to say it can not be recovered in a day.
Lets say the wizard kept the slot but multiclassed into Cleric aiming for healing word, just in case. The wizard has his new spells he can put in his book. He has a 9th-level spell not from either multiclassing nor single class progression. Its an external 9th-level spell.
He should be able, by RAW (though probably not RAI), to learn a 9th-level spell.
Multiclassing did not interfere with his spellcasting feature's ability to prepare spells that he have slots for. Multiclassing, of itself, has no restrictions to what you can know or prepare, it just asks that you pretend you're single-classed into this specific class.
Now, lets say you got an external spell slot not from a wish spell or magic item, but from a different class feature. Lets say you've obtained a mysterious 5th level spell slot as a Sorcerer 7/Wizard 1. Technically, neither classes natively give you this spell slot but you're able to synthesize this slot. Is this not yet another case of an external spell slot being given to a wizard and the wizard being able to now prepare higher level spells than before?
On another note:
A DM that allows a wish from a ring of 3 wishes to give a 9th level slot, even once, isn't "monkey's paw"ing.
He's being absurdly generous.
Wish can't replicate a 9th level spell, much less allow someone to suddenly have a 9th level slot available for use.