In a previous post I mused,
Then I came to feel a few minutes ago that だ becomes な／の in a slight prettying up of statements and explanations. Consider: 〜なの could be from (a hypothetical)〜だの and in actual reality we do know that だから changes to (な)ので. I don't know. Maybe this is a crazy theory. It only really helps for remembering anyways. If you remember things in a circuitous manner like me.
Two points on which I was wrong: だ does not become の under any circumstances, and だから is not conjugatively related to なので. But I am more than ever sure that だ becomes な and that they and the other copula are helper verbs. I just read in one of my books the following:「〜だ」は「の」の前では「〜な」にかわります。
Also something that a little Jeffy bird helped me to realize is that, unless they are strong as nouns, without a な or a だ the 形容動詞s are like half-living lingual things. The な／だ on the end is a little helper verb that gives them life. I spent a long time thinking things like ます and です and だ aren't verbs, but they really do fit most of the description. The only thing is the limits on their conjugation forms.
Anyways, it's just my Western brain's systematizing of it all, but still.
As for my other copula curiosity of late, I haven't found much on the である／だ＝なり front, but I did find this sentence tonight: 時は金なり. Time is money. I imagine なり shows up a lot in Shakespeare translated to Japanese.