Necromancy is like reaching into that same box of knives but fully lit.
*Corection: Can be ...

Its minor difference, but important.

Yes, you Wizard *can be* educated and really well aware of coruptng powers of necromancy ...
OR and that is also possible, that Wizard can simply be some dude who managed few basic cantrips, find some grimoar, and start learning from it ... and before you know it, he ding level 10 and no longer care that his weins starts to show up, his eyes seems watered, his skin is unnaturaly pale, and his muscles seems to be atrofic.

You said it yourself after all ... Wizard "is able" to study, but nothig more.

Fair enough.
but not whatever bizarre aberration they're experiencing now.
To recognize this you would need to metagame ...
Here I disagree and am confused because you refute this argument immediately later in your post. There's no way for your character not to know that what's going on with the tadpoles is an aberration. You should have changed within hours and you haven't.
All they know is that it's not working the way it should and they have some unknown amount of time longer than they normally would.
Yup ...
Educated person can recognize that something is different within meere few hours ... since he is still fully aware of himself, any more time just adds value to this observation.
The question that person must ask then would be: Is that good, or bad sign?
And that is totally us to your character personality ...
- they can freak out anyway, well basicaly for any reason including "simply bcs!"

... just as Shadowheart do.
- or they can be completely cool, simply presume something went wrong and they are perfectly safe ... just as Wyll do.
- and of course, they can do infinite nuances between those two extreme states.

I'll write this off as my nervous, timid brain being fundamentally unable to grasp that people could look at such a risk and not take the cautious route. It seems bizarre and insane to me, but people are weird.
So knoweldge of regular ceremorphosis doesn't really help that much.
Oh please ...
Have you ever seen any educational video about drugs? Dont you know how those people keep repeating the same phrase?
Its: "Im not addicted, i can stop whenever i want" ... and we all know the truth, some of them know it aswell, they just cant force themselves to admit it, bcs truth is too scary.
Same situation is here ...
Yes, some could teoreticaly find out and admit that they have problem ... but for others, the truth is too scary ... so they will rather pretend that nothing bad is happening, and they are fully in control ... until it will be too late.

Same goes on our streets ... speed limits exists for reason, and yet from statistic around 70% of drivers ignore it ... why? Simple, they all believe that they are so good drivers, so nothing bad can happen to them.
In therms of netherese ceremorphosis, your own Ego is your worse enemy.

I see the point you're making, but I don't think it quite links to my point. Also the idea that 70% of drivers ignore speed limits is horrifying and those people should never be allowed to drive. I just took my first driving theory test and it taught me that driving is too dangerous to let so many people do it. Clearly the standards are too low. But that's a whole other kettle of fish.
But you don't actually know what you're losing.
Exactly my point my dear uncolored spirit!

Even one step futher ... we dont even know if we are actualy "losing something" our character actualy had ... bare with me:
As far as i know, Narator only describe our feeling ... so we feel like we loose something that we can never get back ...
That may indeed be part of our self ... or something simmilar, that we loose, bcs we are changing.
OR!
And i think i dont need to say that i find this option more probable, but i just did anyway. 
It can be something spiritual ... or simply collorfull description of remorse ... we know that power is "unclean" ... and yet we used it, we were "pure" before and we lost that purity, forever, with no way to get it back ... like remaining scar on our soul that will never trully heal.
Thats why used virginity as an example ...
In some cultures even today loosing it before mariage is concidered quite bad ... other groups of people simply dont give a shit ... and there are even people who seeing having it as something bad.

And yet it can be described as "you feel like you lost something you will never get back".

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My problem with this argument is twofold. Firstly, if what we're losing is spiritual, that's not any better. This is a magical world with confirmed souls and afterlives. A scar on the soul isn't just a metaphorical issue, it can have very real consequences that may or may not present themselves before death. It's not really an argument for not taking the loss seriously. It's not just metaphorically unclean power, it's potentially metaphysically unclean. Losing one's virginity doesn't actually mark you metaphysically, and if it does we aren't capable of measuring that. But the forgotten realms is a place where such things CAN be measured, more often than not. At least, I have to assume so since we've been given no indication otherwise.
My second problem is with the idea that it's only a "colorful description" because it's describing something that our character is actively feeling. If it's trying to convey remose, then that's the game deciding how our character feels emotionally, which isn't good in an rpg. My interpretation that it's a literal feeling we experience, a tangible loss that can't be ignored and doesn't change based on the kind of person we are. So if we go by your assumption, then we're assuming the game is poorly designed in that instance. My stance tends to be that, unless there's further evidence that a thing is a mistake, I assume it's on purpose to some degree.