Hi - my intuitions fall in the other direction here, so I'm keen to explain why.  Implicit IMOs in front of everything here.

1:
    1.1:  I have a younger brother. My parents could have stopped at one, and my family would broadly still be happy, but my brother is generally happy and leads a good life. Similarly, if they'd had a third child they probably would have been happy and great too, and I would have loved them. All else being equal I wish that youngest sibling could have existed. IMO these two sentiments aren't meaningfully distinct. 

    1.2:  We don't only care about humans.  Sure, the argument for making more humans would apply to insects or something as well. However, most of the things that would kill all the humans would also kill everything else, so for me not letting that happen is still much more of a priority.

    1.3: True on the specifics, false more generally. I don't know exactly what the world should look like, but I'm pretty sure people being happy is good, more people being happy is better, and everything being unrecoverably dead is neutral at most. 

2:

    2.1: If we weren't potentially about to all die I'd be more willing to think about this, but we have to survive the next century or two first. Whether capitalism makes things better or worse for now depends much more on whether it makes us more or less likely to all die, than on anything else (again, for now). 

    2.2: I'm pretty sure non-privileged people also want to be alive and happy. 

    2.3: Possibly, and I'm ok with that. I'd rather live a worse life if it means my grandkids are more likely to survive and have happy ones. Although it's definitely better for everyone to be happier now, I feel like it doesn't amount to much if we all die in the next century. 

    2.4: If I can choose between a surviving but stable society, and a growing one, I would choose the growing one. But both are better than an empty rock, so the priority now is not dying either way.

3: 

    3.1: I'm pretty sure we'll continue to want to be alive and happy, so false. People can't decide what their preferences are, and work to fulfil them, if they don't exist.

    3.2: Our moral intuitions were built for very different-looking societies to where we are today. We like sugar and sex because we were supposed to go for fruit and reproduction; our moral intuitions aren't hugely different. IMO this is in a similar category to people caring more about saving one child than eight of them. 

    3.3: No. 

1.

1.1.: You might want to have a look at group of positions in metaethics called person affecting views, some of which include future people and some of which don't. The ones that do often don't care about increasing/decreasing the number of people in the future, but about improving the lives of future people that will exist anyway. That's compatible with longtermism - not all longtermism is about extinction risk. (See trajectory change and s-risk.)

1.2.: No, we don't just care about humans. In fact, I think it's quite likely that most of the value or disvalue will come from non-human minds. (Though I'm thinking digital minds rather than animals.) But we can't influence how the future will go if we're not around, and many x-risk scenarios would be quite bad full stop and not just bad for humans.

1.3.: You might want to have a look at cluelessness (EA forum and GPI website should have links) or the recent 80,000 Hours podcast with Alexander Berger. Predicting the future and how we can influence it is definitely extremely hard, but I don't think we're decisively in bad enough of a position where we can - with a good conscience - just throw our hands up and conclude there's definitely nothing to be done here.

2.

2.1 + 2.2.: Don't really want to write anything on this right now

2.3.: Definite no. It just argues that trade-offs must be made, and some bads are worse even than current suffering. Or rather: The amount of bad we can avert is greater even than if we focus on current suffering

2.4: Don't understand what you're getting at.

3.

3.1.: Can't parse the question

3.2.: I think many longtermists struggle with this. Michelle Hutchinson wrote a post on the EA forum recently on what still keeps her motivated. You can find it by searching her name ont he EA forum.

3.3.: No. Longtermism per se doesn't say anything about how much to personally sacrifice. You can believe in longtermism + think that you should give away your last penny and work every waking hour in a job you don't like. You can not be a longtermist and think you should live a comfortable, expensive life because that's what's most sustainable. Some leanings on this question might correlate with whether you're a longtermist or not, but in principle, this question is orthogonal.

Sorry if the tone is brash. If so, that's unintentional, and I tend to be really slow otherwise, but I appreciate that you're thinking about this. (Also, I'm writing this as sleep procrastination, and my guilt is driving my typing speed)