Several of these really only apply to a fully socialist fully centralised system, I'll answer under that assumption.

consumer choice

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Instead of this. Where dozens of different companies are all independently pursuing exactly the same thing, wasting the labour of thousands of talented engineers competing to come to exactly the same outcome, you will instead get less of this skilled labour wasted. It takes the labour of one team of engineers to come to this outcome, all the rest of these talented people can be put towards achieving something useful instead.

This parallel construction of the same product that does the same thing with practically no difference is a monumentally huge waste of labour. One of the easiest examples of the inefficiency of markets. Think of what useful and innovative things these people could be doing instead of.... This? Making the same shit over and over and over in competition with one another.

How would niche things, that benefit some part of the population but not everyone, be produced? Things like... fumo plushies, board games, or to put a less banal example, something that helps a condition that is uncommon and doesn't spread but still exists, like special shoes to help some kinds of foot deformation for example...

There's no reason niche things can't be produced by a centralised system where inventors bring their things and make a case for them, then the department assesses their usefulness and agrees or does not. For niche things this is actually much more likely to happen than for things that aren't niche, because it costs considerably fewer resources to do.

Lots of completely useless products will never get invented, simply because there is oversight.

Section 19 I'm curious what communists think about this with a modern lens? AFAIK a revolution in a single country did happen right? And in Russia so none of the places Engels proposed. It didn't really spread from there.

If the German revolution had succeeded the global revolution was extremely likely. All of Europe, the middle east and asia would have gone to revolution. Africa would have followed easily under these conditions. South America too. The US would have been isolated and followed.

It's not often in history that killing a single person changes its course, but in the case of the German revolution the killing of Rosa Luxembourg did.

do marxists think only economic class exists? wouldn't there still be political classes? here it says that classes would end up disappearing because they only form due to division of labour. But isn't there even in a fully realized socialist state a division of labour? even if everything is nationalized, isn't there still a difference in power between, like, a furniture factory worker and the bureaucrat that oversees the state's furniture building company? even if that bureaucrat is not monetarily richer than the worker per se.

Class under capitalism is used to interpret the primary contradiction of capitalism, class war. Between the exploiter and exploited. You could divide up the new system into a hierarchy of power to analyse its contradiction dialectically and try to discover where things will go but you're flying blind with no historical examples due to the nature of capitalism still existing. We simply don't know what contradictions will arise in the new society. Certainly history will not end, but this is guessing best left to fictional writers as it is so far in the future to not matter to us.

I have one doubt about what Engels says about democratic socialists, mainly that small capitalists ("petty bourgeoisie") in general tend to have the same interests as the proletariat.

He's right about democratic socialists, but you're right about the petty bougs. The modern interpretation of this class is that they do not hold the interests of the proletariat, and they are the primary class that pushes for fascism.