There are two kinds of plagiarism:
- of ideas / scientific plagiarism (fully solved by a citation of the original work). Buffy's answers here and on a followup question have some good insight into why this is such a big deal (e.g. for future scholars wanting to know the scientific context in which something was written).
of the wording. Taking credit for the wording doesn't hurt the scientific value of the work (and thus is less serious), but still makes the author look like a more skilled wordsmith than they are. (Unless they do it badly/clumsily; see @Ben's answer on the followup.)
Aside from plagiarism (taking false credit), this also violates the https://en.wikipedia.org/wiki/Moral_rights of the creator, specifically the right of attribution of your wording. Some countries give legal weight to the moral rights of the creator beyond just the usual fixed-term / dollar-value copyright. (And if the work was published, could also be traditional economic copyright infringement)
Citing and then slightly paraphrasing instead of quoting with quote marks implies that the idea is from the cited work, but the wording is fully original. If that's not the case, it's a violation of the moral right of attribution of the original author for the wording, as well as plagiarism.
And if published, also becomes an issue of traditional copyright violation and suing for damages. If that's not the case, and only the moral right of the author have been violated, I'm not clear on what if any legal penalties there might actually be. And it's still plagiarism, which is a separate thing.
The student has violated your moral right of attribution for your wording of the explanation. This is probably a large part of what's bothering you so much, not (just) the plagiarism.
Perhaps you didn't get anywhere when contacting people if they weren't clear you were not talking about "scientific plagiarism" (lack of citation), which is the much more serious issue that would definitely warrant contacting people up to and including the head of the university.
In hindsight you might have had better luck if you could start fresh after reading these answers, but at this point I think the ship has sailed on getting the passages actually quoted, or more fully rewritten (at least as an erratum). Or at least an apology, which might have been all that was feasible given the restrictions on (not) modifying university archives after the fact, even to fix errors.
(I wrote the rest of this answer before looking into moral rights as something separate from copyright and plagiarism.)
Plagiarism of wording is a different kind of offence from copyright infringement: the victims are people that are tricked into being impressed by the person's ability to express ideas with words, when in fact it wasn't actually their word-crafting ability that resulted in the clear explanation found in the work they wrote.
In copyright infringement, you would be the victim, and suffer damages from someone else profiting from your creative work. (Presumably instead of you, this is the main point of copyright law.)
Since they cite the work they're paraphrasing, they're not trying to hide the source. Anyone who looks will see that their wording looks like yours, but yours must be the original because they cite you.
The student has done something wrong, but it's hard to find any way to say that they've wronged you directly. Their writing doesn't diminish your reputation, it just makes you unhappy with them for trying to take advantage of your work to achieve their goal (getting a degree?) with less effort than they should have needed. i.e. trying to cheat the system, not cheat you.
Keep that in mind when pursuing "justice". It's obviously more personal to you when someone passes off your words as their own, with only minor rephrasing, but in terms of any academic or legal consequences this is probably not too different from pursuing a random stranger for littering in a public park. Or maybe for running slightly inside the lines in a race (which you weren't competing in) at a sporting even. Or pick any minor offense where society / the public is the victim.
I'm talking about plagiarism of wording like you discuss, not of ideas / scientific data / results. Again that's similar in that the original source isn't damaged in a legal sense by the plagiarizer, but it does much more damage to science as a whole than passing off someone else's wording as your own.
To help separate plagiarism from copyright infringement, consider a case of someone trying to pass off a piece of old public-domain work as their own.
It's still plagiarism even if nobody knows who wrote the original passage. e.g. an anonymous work, or something like a folk tale / oral tradition.
Let's take Shakespeare's writing as an example (and pretend that it wouldn't be recognized right away by someone). They aren't harming the bard. (Well you could imagine slight indirect harm if they end up associating Shakespeare's phrasing of something with clumsy un-enjoyable work, so people enjoy the original less when they read / hear it.)
But even if they directly harming Shakespeare's reputation, that's not what plagiarism is about: it's about taking undeserved credit. It's not solely up to the person who's words were copied to enforce anything, it's up to anyone who notices or is made aware to call out the plagiarizer. If the plagiarizer is working on behalf of anyone (e.g. a speechwriter), they might be fired for making the person giving the speech look bad.
(Or before publishing, up to their editors or anyone else to stop them from trying to commit accidental or intentional plagiarism.)
In your case, the people you contacted should have got the student to rewrite or actually quote the passages. I think you've done your part by calling attention to the plagiarism. If you want to publicly call attention to it in some way, that would be the last step that would be worth taking. But honestly it does sound like pretty minor plagiarism, and making too much of a stink would (unfortunately) be more likely to make you look petty and vindictive in some people's eyes.