It's definitely an interesting question. My strong suspicion is that gynandromorphs probably have an intermediate level of sex hormones and even if the organs were properly developed this would tend to prevent fertility but it's not like I've checked.

I don't know for sure, but thinking about how it works for humans I suspect that in many species the hormone levels would be weird for a typical individual of either sex and that might cause infertility. It's usually pretty hard to prove a negative though, I wouldn't be utterly shocked if something fairly basal like a frog might have one set of gametes or the other come out functional.

This sounds so cartoonishly evil that it's difficult to believe, but people have actually been deliberately injected with human cancer cells and they generally do NOT get established.

Horrible but true:

Tuskegee syphilis study not America's only medical scandal https://aquila.usm.edu/cgi/viewcontent.cgi?article=1242&context=ojhe

In general it's going to be ok because a healthy immune system is going to recognize the cells as foreign. The problem with getting cancer for the most part is that they ARE your cells. (Ideally your body recognizes your own cancer cells anyway but you get the idea.) If you're immunosuppressed for whatever reason it's possible.

Among other things the basal metabolic rate of someone who's been exposed to the cold regularly for a while rises, and they tend to put on additional brown fat which produces heat. It might be interesting to see if the response is stronger in populations that have existed in cold places for s long time vs say, a population that lives near the equator. I suspect that there isn't enough time to evolve huge change in the capacity to alter the strength of this response though and people are actually really good at causing gene flow through all their different subpopulations even in antiquity.

You'd have to define deep I suppose. I think it probably IS fair to say that most fossil seabeds were not from what we'd call the Hadal zone nowadays.

Hmm. Actually, they have a pretty good idea of which plates were where at different epochs and I'm not sure there IS an exposed rock face that would have been Hadal, and I'm not sure that geologic processes are particularly likely to result in present trench communities to fossilize and then be exposed some time in the future. My general impression of most of the truly deep sites is that they're rifts at the bottom of subduction zones and rock that ends up sinking below a plate isn't coming out looking like it did when it went in. I THINK. I study evolution but not paleontology or geology. If someone does know of one I'd love to read about it!

You might find this interesting:

https://www.jstor.org/stable/20144263

That's a paper on a fossil anglerfish. Not all anglerfish live in the deepest parts of the ocean, but apparently they think the formation that one is from represents mostly fish from around 1000m. It certainly gets deeper than that but a lot of what you'd see living there would be "weird" compares to the fish most people are familiar with.