Converting

[Paleophyte]

Yes, it really is. Aside from adding a few hundred cc's of brain matter you'll need to produce a whole shitload of hellishly complex and entirely new neural systems. There's simply no doing that without a few thousand generations of intermediate steps. You'd have better odds of going from a hand-drawn cart to a Porche in a single leap.

I don't think you're really addressing the issue here. The modern-primitive dichotomy is logically forced

You mean mythologically forced. You're trying to rewrite fact to fit your narrative. There is no logical necessity for the dichotomy.

if it's hard for you to imagine where the line had to be drawn, it's a lot easier to do so for a deity.

Not hard, impossible. There is no line that can be drawn between ancestor and descendant. They are of the exact same species by definition.

Porche wasn't built in a single leap of course, but you would still have a manufacturing date.

The date is immaterial. You wanted to build a modern human brain and mind with a single, small change. Not happening.

Our closest ancestors surely had a similarly complex brain, but there surely has to be -logically- an infinitesimal increment where we're justified to draw the line

No there isn't. There is absolutely no logical necessity for reality to conform to your notions or attempts to cram things into pigeon holes.

To sum it up, you're arguing from ignorance here, we don't really have a lot of details about all the intermediate steps that took place in the distant past.

No, I'm not. I'm arguing from the certain knowledge that there was no dividing line between any ancestor and their immediate descendant. What you're arguing for is something that is for all intents and purposes non-human giving birth to something that is human. Evolution don't work like that.

Another way to put it is this: you have very low IQ and very high IQ people in this world, cognitive ability between the two groups is simply huge

Diversity within a species really isn't going to be your friend here. All you've managed to do is add yet another dimension to have shades of grey in. It ffectively guarantees that any "Adam" had a large number of ancestors who were sparkier and more worthy of your deity's attention than he was.

And the anatomical brain differences may not be more than a difference of number of synapses per neuron, which can be prompted by a small "click" or stimulation of the synaptogenesis process.

Synaptogenesis and other small changes won't get you diddly without the complex neural structures already in place. More connections doesn't help if you don't have speech centers.

So the anatomical difference between Einstein's brain and, say, his mom's, might roughly be an example of the gap we're looking for between modern and pre-modern human beings.

A lovely example given that both Einstein and his mother were obviously human.

Another example to clarify my point : world war didn't really start by a single bomb explosion, or some assassination, it came about through almost a decade of smaller conflicts and diplomatic tensions, but we still have starting and ending dates of the event.

Congrats. You've convincingly demonstrated that humans excel at inflicting arbitrary boundaries on reality. Which side of this are you arguing again?