building the data center on the moon where everything is cold and the moon is a big heat sink.
The moon can get surprisingly hot! 120 degrees celsius in some parts which is enough to boil water at atmospheric pressure. The cold parts of the moon are “craters of eternal darkness” which have a stable, extremely low temperature so one might be able to use those.
Also, I’m not sure to what extent the moon can be used as a heat sink, given its low thermal conductivity. Then again, if your surroundings are in the tens of kelvins, that might be good enough on its own.
One nitpick is that the a part of the immune system (your population of B-cells) can rewrite its source code between generations, and surprisingly rapidly! In fact, because their goal is to produce antibodies which grab on to pathogens, B-cells will actually mutate the genes encoding for these antibodies at an extraordinary rate. And, they’ll reproduce more the more their antibodies are shown to work (that is, to bind to a piece of a pathogen!) This allows your body to run evolution far faster than a lot of microbes, which have selfish genes that “want” to be passed on without mutation.
Now, your main thesis still remains. The adaptive immune system, which includes all B-cells, is only found in vertebrates. The majority of the animal kingdom does have an immune system which is specified once, and they get by just fine. However, it’s also worth noting that the largest and most complex animals are ~all vertebrates, and this might have something to do with the immune system, among other things.