This is the only one of your points I want to comment on.
No doubt - Chaos has within it Space Marines beyond the original traitor Legions. I have in my hand the current edition codex for CSM, and I am reading examples of this as I type this post.
At the same time, I can't help but also read about characters such as Kharn the Betrayer, who in an adjacent article is depicted as a blood-crazy warrior who kills five of his "Brother-Slaughterer's" because - one corpse in the name of Khorne is as good as any other. There are tons of examples making the explicit point that while Chaos is still deadly, they are far from what they once were.
- Angron, Primarch of the World Eaters, led his Legion of 50,000 Space Marines in a rampage lasting two hundred years across 70 sectors in an event called the Dominion of Fire. This was in M38 - Angron had far more than 50,000 Space Marines before throwing in with Horus. And Angron did not finish the Dominion of Fire with 50,000 Space Marines - no doubt he took casualties.
- And keeping with Kharn and the World Eaters is the event at Skalathrax, where indecision amongst the World Eaters while fighting a campaign of genocide against the Emperor's Children fragmented the Legion forever - since Skalathrax, there has been no World Eaters Legion - just the Khorne Berserker. Their numbers are only diminishing. With a breakdown in organization, more things are going to slip through the cracks - this means gene-seeds are forgotten and left on the battlefield.
- The Thousand Sons started weak and only got weaker. True, Magnus was the first Primarch and was with the Emperor since Terra, but mutation among his Legion led to it's near destruction even during the Grand Crusade. And then the Space Wolves wiped out their homeworld of Prospero and many Thousand Suns in the process. And then, after the fall to Chaos, again mutation among the Legion's psykers thinned their ranks until finally Ahriman came up with a solution - but this left the vast majority of surviving Thousand Suns as mere shells - their bodies are destroyed, only their armor remains. This means there is no gene-seed to salvage. By the time Ahriman enacted his final solution, the lore doesn't support Thousand Suns having more than maybe 5,000 members, maximum. And this was back in M31, and we can assume they've taken casualties in the ensuing 10,000 years.
- The Emperor's Children and Night Lords are barely worth talking about in terms of sustaining the Chaos war-machine - they never had the numbers to be anything more than additions to a larger force. The Emperor's Children were born with a defect that dwindled their numbers to sub-Legion size, forcing them to accompany Horus's Luna Wolves in the early days of the Grand Crusade - and after losing themselves to Slaanesh, it's only been downhill. The Night Lords similarly lost their **** after Conrad was assassinated, pulling a World Eater and fragmenting into warbands and mercenaries. More disorganization, more dysfunction, more cracks.
- Mortarion doomed his Death Guard with service to Nurgle. The Destroyer Plague is as likely to kill a Space Marine as it is to create a new Plague Marine - hardly an efficient means of recruitment. The lore says Typhus and Mortarion had a falling out and split - more likely Mortarion sent Typhus away to keep the Destroyer Plague from completely tearing the heart out of his Legion. It says right here that Typhus was the only survivor of his ship that had been infected with the plague - what happened to everyone else? Obliteration, that's what.
- When you read about Huron Blackheart, it's in the context that he's completed another daring raid and his army of followers has grown. What the lore doesn't hide, but still fails to emphasize is that he isn't recruiting patriotic Imperials - rather he's picking up stray Chaos warbands. This is kind of cheating - because Chaos isn't growing, it's just falling into line behind a leader so it may be counted. Yea, Space Marines are betraying the Emperor in ones and twos, but that's hardly a flood, more like a trickle, and not enough to stop a string of bad luck from tearing Blackheart's 'empire' in half.
- Lastly, I want to talk about Abaddon. You don't get his full picture until you've read about everyone else - he is only as strong because he has recruited everyone else. The Khorne Surgeons needed to make more Berserkers? Abaddon hired them. The Thousand Sun sorcerers? Abaddon bought them. The Night Lord Raptors? Abaddon. Abaddon did not go on a massive PR campaign to recruit from Imperium forge-worlds, swaying loyal Space Marines to his cause. Rather, Abaddon went to the bar every CSM was hanging out at and bought them with gold. Think less 'Alexander the Great of Macedon', and more 'Captain Jack Sparrow in Tortuga.'
Chaos is not growing. Chaos has never been growing. After Horus was defeated, loyalist Space Marines undertook "The Scouring" in which... I'll quote it in it's entirety:
The Imperium kicked Chaos's butt, corralling them in the Eye, and forces them to go up against crack troops if ever they want to take a peek.City-by-city, world-by-world, the forces of the Emperor reclaimed the galaxy once again from the armies of darkness. The Space Marines loyal to the Emperor fought and fought and eventually hounded the Traitors into the 'Eye of Terror' - that strange area of space where warpspace and realspace are bound together. This bizarre region became the prison of the traitors - and their greatest stronghold. Guardian fleets patrolled its borders and nearby planets became garrison worlds ever ready to combat raiding forces from the 'Eye'.
Within the giant warpstorm, the traitor Primarchs and their Legions found sancutary amongst the nightmare demon worlds. They warred amongst themselves for dominance and territory, while factions split from the Legions to found other warbands and armies. Thus started the struggle that has lasted for ten thousand years and even now besets the Imperium.
No, Chaos is not growing. What happens in the Eye does indeed stay in the Eye - the problem is not much entered the Eye to begin with, and they've only been tearing each other down since they arrived.
This thread is about whether the Imperium is shrinking or growing. If the Imperium is shrinking, I hope I've made the case that it isn't Chaos that's causing it. They have their own problems to deal with first.