I live in China and my Chinese is pretty good (I study a degree in economics that is fully taught in Chinese), so I think I can fairly solve your doubts.

Do people in China have to boil their water before drinking it? I’ve found sources dated as recent as 2012 saying this

Yep, even in big cities like Beijing. Actually, most people just buy mineral bottled water.

If I were to move to China, what western media would I have to be prepared to not have access to anymore?

Pretty much all western news outlets and social media. You will also have problems accessing certain platforms like Steam, but you can always get a VPN.

Just as one example, could I still follow/listen to American & European rock/metal bands?

Of course, no problem with that.

After several years doing jobs that I didn’t like very much, I finally recently moved to a job in an area that I like a lot (renewable energy). How easy would it be for me to find similar work in China?

Being completely honest with you, if you can't speak Chinese, and you haven't graduated in a Chinese university, the chances of getting hired by a Chinese company in your sector are very small. You have to think you are competing with thousands of Chinese engineers that won't bring any trouble (language, cultural differences, visa arrangements, etc), so unless you have something special to bring to the table, they probably won't want you.

Sometimes it seems like the only work Americans can find in China is as English teachers.

That's a sad reality, but also an understandable reality. Why would you hire a guy that can't even speak Chinese and only brings troubles, when there are thousands of local engineers you can chose from? Would we expect a company in the US to hire people that can't speak English in high paying positions? Even foreigners like me, who will graduate from a Chinese university, will probably just be hired in some department related to international commerce.

If you want to get a job in China without a good Chinese level and a degree in a Chinese university, your best chance is to be a teacher, or get hired in some American company that operates in China.

Why does China accept so few immigrants? (500-2000 per year, vs about 1,000,000/year accepted by the USA, and 600,000-2,000,000/yr accepted by Germany)

China only wants high value immigrants.

Regarding sex, dating & family, is China generally more liberal than the US, more conservative, or about the same?

Very conservative. You better not try to have one night stands or have a "temporal" girlfriend/relationship, that's typical sexpat behaviour. In Chinese society it's expected that your girlfriend or the person you have sex with is the one you will marry, so make sure if you have any relationship and you don't intend to commit, both of you are in the same page, otherwise you would be taking advantage of those cultural differences.

Does housing work in China similarly to how it works in the US? How much of their income do Chinese people typically spend on rent/homeownership?

Depends on the city, in big cities it's very similar to most developed countries, while in smaller cities it's more easy to find more reasonable prices.