Like you were saying, Kojima killed-off Liquid too early in MGS 1, and his persona was kept alive through Ocelot, to ensure that he was a recurring villain. Imagine how awesome it would have been if it were actually Liquid's ghost, and not Ocelot going crazy...
He killed him off so fast because maybe he wasn't sure if there would even be a MGS2, so he went all out with trying to tie things together. I mean he did give a slight cliffhanger ending but MGS1 is pretty self contained.
Ocelot was the puppeteer in MGS2 and could of been again in MGS4. But him turning himself into Liquid makes no logical sense at all, he didn't need to be him whatsoever
Is there a clear explanation in MGS4 as to why he chose to do that? Been awhile since I played it. IIRC wasn't it supposed to be a distraction so Snake could take the AIs out?