The important bit in the post was the pessimistic setting. Everything in 40k against it's worst forms with things doing their damndest to get them to that form.
Democracy is not a better option in this setting because it will often be bastardized into a hellhole as bad as what you want it to replace. It being a feature of the world is fine and dandy but actively trying to export it is going to get slapped by the powers that be since the Imperium is fundamental an ABSOLUTE monarchy.
Especially if people want the chapter to actively cause democratic rebellions which is something they will get them declared rogue or outright traitors for.
Yes, but that isn't always the case. Look at Keepers of Truth, people pointed out how awful of a idea is resurrection of Imperial Truth, yet, the worst didn't happen.
Also, no. Imperium, at least now, is not absolute monarchy, the authorities of wider Imperium, Inquisition and High Lords, don't give a fuck, what's the political system on a planet, they care about the tithes and nothing more.
As long as planet remains loyal, gives tithes and some major fuckery isn't happening, the wider Imperium doesn't care. For example: Let's say that uprising happens on the world Birb V, the rebellion is against the specific planetary governor, not the Imperium itself, the uprising succedes, previous governor is executed and the rebellion leadership takes control, some planetary laws change but nothing more, so the wider Imperium accepts the new leadership, it's proclaimed to have been Emperor will and that's it.
Rebellion which are against Imperium are the problem not rebellions in general. And again, Emperor is currently the Deity first and foremost not active Monarch.
So it's perfectly possible the Chapter wouldn't have any problems by changing planetary political systems and if they make worlds more efficient, the Chapter will be appreciated.
You have made three errors in this sentence.
First: Frog did not write that addition to the planet's description. I did.
Second: That is not a Word of God (or Word of Saint Paul, I suppose). It is Grimm's in universe opinion.
Third: Grimm was not saying he thinks "democracy is vulnerable to Chaos" is false. He believes it is false for this specific planet, because said planet managed to remain sane during the Heresy.
Didn't know about the first, rest is fine and thank you for the answer. This still proves that democracy doesn't have to be easily corruptable and the Chapter of Word Bearers will be spreading the culture, specific form of democracy practiced on the planet and the ideas of the planet people, so the planets influenced by the Chapter still wouldn't be easily corrupted, at least not more than any other political system or world.