However, should Kimi have pushed LH immediately out of the way to get on the dry line...?
I don't understand your point about lines, once Kimi was ahead he had the advantage and could take whatever line he chose, couldn't he?.
Could He? How? LH was next to him on the dry. What I remember, he threw his car inches in front of LH´s nose just a couple of feet before the next turn in an attempt to get it on the ideal line. I don´t see how he could have done this earlier, as LH was next to him almost till the end of the straight. And then Kimi did a very strange manoevre, just before LH past him (thanks to this manouervere (damn, how is it spelled?))
This manouvre seems to be caused by either the rapid lane switching on the wet (he lost the car). But mayve even he got out of the way to prevent LH crashing into his diffuser. Remember how Verstappen crashed into Montoya when he threw his car in front of Verstappen just before a corner, and Verstappen totally lost grip due to turbulent air? I did not hear Kimi about all this yet, but this theory (which I made up now), would not surprise me either.
The speed difference was 6 km/h. Do you know how many seconds it takes to get a "safe" 6 m distance (the length of a car + a little space)? it takes 3.6 seconds at 6 km/h. 3.6 seconds at 300+ is 300 meters, roughly estimated. How long is that straight? It took 300 meters to get past him at that speed difference. How much smaller was the difference before LH lifted?
Still, I would like to hear what Kimi has to say about this. But we never will, because Ferrari will not let him...
Having looked at the youtube again several times (worth doing incidentally), I'd now say that LH was never in the slipstream and certainly never gained any slipstream advantage. He might have wanted to, but Kimi was weaving the whole way and I would guess that was designed to prevent LH getting in the slipstream.
However it's also clear that LH was behind right up to the braking point and Kimi chose the outside line - he could have gone straight for the apex if he'd wanted to. Perhaps he thought LH would overcook it going for the inside and he could retake him on the exit. Or maybe he just didn't fancy the inside line himself. Who knows, but if you look at it, it's clear that Kimi could have taken the inside if he chose.
Incidentally the straight is only 300m from the start/finish to La Source, which is roughly when LH let him past, so I can't see that slipstreaming would have been a huge issue anyway at the speeds they can do in such a short straight.
I actually think Kimi is a better driver than LH, but LH was all over him because he had more grip. Having watched it again, I must say I now think the argument that LH gained an advantage and that was what enabled him to overtake is actually a lot weaker than I thought before. He overtook on braking owing to more grip, that's what it looks like.