I think if Sanguinius fell, big E would turn into the Dark King in order to defeat them or he wouldnt go to the vengeful spirit altogether

Khorne would become lesser demon of Sanguinius.....

If this was the sole thing that changed, then yes I agree this is how it would play out, and you did a good job highlighting the dramatic irony of it. I think a few more things would happen though, because the Chaos god's Great Game comes into play. The World Eaters and especially Angron would challenge sanguinius for his chance to be representative of Khorne. They would have a civil war of sorts, potentially requiring the intervention of other traitor primarchs. This would potentially draw traitor forces away from Terra for the siege, allowing Guiliman the extra time he needed to get around the Ruinstorm. Fate is also rather fickle and often symmetrical in warhammer, so I believe that if Sanguinius were to be present at the final duel against Horus, so too would Rogal. Would the emperor still lose? Likely. But I think it would end in a draw where both Horus and the Emperor die together. The loyalists still win, but there is no Astronomicon, and the Webway cannot be contained. Vulkan will have to activate the Talisman and destroy Terra. The setting is MUCH darker both literally and metaphorically.

you say the same thing several times, it's too much