You have it the wrong way round.
The Heresy occured, attrition went up, rate of recruitment, and short cuts were made to get people battle ready.
Where once you had mighty, honourable great crusade marines, you were now left with bitter, hollow marines who were but a shadow of what they could have been.
Zerachiel, as far as I'm aware, that fluff is considered retconned entirely, useful only as taster fluff to the Horus Heresy background information laid out in the FW productions, which outright refute such information provided within them as lies and contradict at other times.
The need for Space Marines was coming to an end. The Emperor had no need to perfect it. With the Great Crusade theoretically complete, and everyone converted to the Imperial Truth, and with the belief of the Chaos Gods wiped out, what need was there for Iconoclasts like the Word Bearers? What Cities needed destroying by the IVth? Or letting slip the World Eaters? Or the Night Lords? Horus' own legion would have proved useless. The Lion, Russ, Sanguinius, very little were they needed for; arguments could be made for Sanguinius re Imperium Secundus, but honestly Imperium Secundus needed a Warrior-Emperor and avenging angel figurehead; that was he; Imperium Primus needed no such thing. The Thousand Sons would have had to have been killed at some time, as they continued to practise Warp Magic, and the veneration of the Emperor as a god would have been dangerous, so that is 8 legions at least wiped out. The Khan hated the yoke of the Imperium, but his legion could have been relatively peaceable, but he'd have "rebelled" and fled eventually, so that's 9.
What does that leave us with?
Emperor's Children; maybe, they were reasonably cultural, keep
Raven Guard; no, these would have been nixed
Ultramarines; notable for administration and being relatively peaceable with a geneseed noted as a calming influence, keep
Salamanders; champions of humanity; keep
Iron Hands; nopenopenope
Alpha Legion; who the fuck knows? Whether they wanted to kill them or not they'd stay. To save the trouble, I'd say keep them
Imperial Fists; Praetorians, but warrior minded (as opposed to the 4th necessity as soldiers, but following Pertys lead, architects at heart initially), keep
Death Guard/Dusk Raiders; nope
So there you have it; out of the possible ones who would have a place in the new Imperium post Great Crusade, you'd have 4, maybe 5 legions worth keeping, and not necessarily expanding. The only reason I could see to keep them would be to a) power the Golden Throne with their psykers, b) get into the webway and fuck shit up.
After then? Who knows; he didn't foresee the Heresy, but that's Chaos and funky shit, but with his astronomicon, it's questionable whether he even knew of the intergalactic threat of the nids and any potential need to have the legions around for those purposes.
While I totally agree with a lot of your analysis of the legions how about the Blood Angels, Sons of Horus and Thousand Sons? Sanguinius was a well known figure in the Imperium and as far as I know he was widely loved. How would it seem if his Legion was disbanded?
The Sons of Horus would be even more politically charged. Would the lead Primarch accept having his legion disbanded? I can't see it happening. The Thousand Sons I could see being kept to assist with the astronomicon and Golden Throne.
Also how about the first Legion, the Dark Angels? Lion El'Jonson and the First Legion seem to have held quite a lot of political influence which is why they were able to get weaponry and unusual relics without anyone batting an eyelid. They're also pretty balanced so I can see them staying too.
I think the Imperium would also need the Raven Guard, after all even empires at peace need special ops troops so if Alpha Legion stays then I think the Raven Guard would stay too?
Re the info from Index Astartes, I accept a lot of it has been retconned but I can't find any info stating the info on the Emperor's Children having all their implants working at maximum efficiency has been retconned. As far as I'm aware retconned is where future information is released which contradicts earlier information and I can't find anything that contradicts the info from Index Astartes. I may be wrong and if you can cite your sources I will accept it.