assume Spell Knowledge prereq is altered to include divine casters as well (because Archivist exists)...
Ok, we can assume that... it won't be true, but we can assume that for whatever good it does.
I'm not sure what Archivist has to do with it. Archivist is a divine spellcasting class. It casts divine spells. It operates like a wizard in that it can add spells to its spellbook, but only if they are divine spells. It is thus able to with a little bit of cleverness add spells from any divine spell list, including divine spells that are normally only available on domain spell lists and so forth. But they are always divine spells, and the prerequisite is what it says it is: "Ability to cast spells of the maximum normal spell level of an arcane spellcasting class." Thus, being a 20th level Archivist wouldn't qualify you for this feat.
and you pick a class to add the spells to.
You cannot just pick a class to add the spells to. You must pick the class that you have already obtained the maximum normal spell level for that arcane (now divine) spellcasting class.
Could I pick: any divine spells, just cleric spells, just spells on a domain somewhere, just spells on a domain the character's deity can grant?
You can pick any spells that are on that classes class spell list.
You can take this feat with Warmages, the author of that class even said so in an article about the epic warmage.
Of course it "works" with Warmages. Warmages are an arcane spellcasting class. You'd have to be at least an 18th level Warmage, but it works with Warmages. The real question you are asking has nothing to do with whether Warmage meets the prerequisite, but whether it allows Warmages access to spells that aren't on their class spell list. I'm guessing that it does not, since none of the optimization handbooks are suggesting it for the Warmage, but instead are pointing to the usual suspects like Arcane Disciple, Runestaves, Rainbow Servant, etc. In general, a class is not able to cast spells that are not on its spell list. I don't know what article you are referring to. I can't find it online.
I'm pretty sure in order to do what you want, the wording of the feat would have to be different - something like: "Add two arcane spells of any level to your class spell list. Those spells are added to your list of known spells."
That said, you are trying to kludge into what sounds like a 3.5e game a 3.0e Feat, and so it doesn't sound like you are interested in rules so go ahead and make it work however you like. If you want "Spell Knowledge" to allow you to add any spell from any class list and you want to ignore the prerequisites, go ahead.