You are running into the problem of Occam's Razor, aka Parsimony, and others...
https://simple.wikipedia.org/wiki/Occam%27s_razor
https://evolution.berkeley.edu/evolibrary/article/phylogenetics_08
These ideas basically tells us that the path with the fewest steps, is that the path that is likely to happen.
So the first problem is that to get to where you want this species to be you require so many contrived circumstances for a benefit that isn't even really that good (diamond skin?), that it is extremely unlikely to happen. It is not realistic, because if a real organism needed this amount of defense (diamond shell/skin) there is almost certainly a better, cheaper, more efficient, and more advantageous way to evolve a comparable benefit. For example if the problem is predators, have extremely dense bone? Be faster to not get caught? Etc.
So you would need a ridiculous number of very specific circumstance to get to the point where what you have suggested could even be possible. But after it is, you still have problems with this species that would basically guarantee it dies immediately. For example what happens when the organism grows? Diamonds don't really expand to fit the shape of the growing animal underneath. So you have created an animal will likely go extinct in its first generation as it is crushed by developmental limitations, unfortunately. But this is not even the biggest problem.
The biggest problem is the radiation. You're talking about having microwaves passing through this animals body that are so strong they turn gas in the womb into plasma. Nevermind the problem of containing plasma. Any animal that uses DNA will never be able to survive the radiation, or make a thriving species with that limitation. Any baby using a genetic coding system (even if it is different from dna) that can be destroyed by intense microwaves everywhere around them, microwaves that are strong enough to create plasma in its womb, would also have this coding system get destroyed. Think immense dna damage or even just have the rest of the animal turn to plasma too, why only in the womb and not all of its flesh? Regardless a baby that has to develop from a single cell or a small relatively unprotected embryo at some point in its developmental cycle will not survive any of this process.
So is this realistic? It is not. Even in a crazy sci fi setting this is still not very realistic. But somehow somewhere if you think hard enough, there might be some way for this thing to exist. But you are looking at thousands of very specific environmental pressures.
For this one you are probably going to need a lot of hand wavium.
