Reality Check time.
In my world there is an old human guarding something of great significance.
Presumably the least assumption we can make is that this guard was put there with the intent of surviving by whoever put them there. So the first problem is that we would assume the system was intended to keep them alive and failed for some reason.
If, BYW, you mean an "old human" meaning a human who was old when they were frozen, keep in mind that if I want to go to all this trouble to guard something I'm not going to freeze someone old, I'm going to freeze someone at the peak of their physical powers - that's e.g. late twenties special forces types, not some retiree looking to beat death. In the unlikely event this corpse is revived I want them able to kill, main or intimidate anything I can imagine. They'd be equipped with enough firepower that I feel is needed to counter any threat I consider reasonable. They'd also have enough engineering skills and knowledge to e.g. repair stuff and check it's all working. So for "old human" replace with "volunteer Captain of special forces with PhD in engineering". Think "astronaut" level of skills and knowledge. You wake this person up, you're gonna regret it. :-)
All chemical reactions in their body have been slowed down to a microscopic fraction of what is deemed usual.
Handwavium physics that means you've broken the "reality check" principle.
Freezing things can't slow down reactions in that sense. What it does is lower the energy levels that particles occupy to the lowest possible distribution of states. This won't slow things down, it will completely change the nature of the chemical processes taking place. You don't get the same chemical processes slowed down, you get different chemical processes, and in particular you get all the nasty effects that means you do damage.
And undoing that isn't the same as doing it in reverse. We have no idea how you undo this and you need both.
Once the peace of the chamber is disturbed, this condition will be broken until the threat is dealt with.
So your handwavium method seems to require that when they are handwavium revived they are also handwavium wide awake and alert and able to instantly work out the nature of the threat and defend themselves. That's not how humans work. Wake me or anyone else up or drop me into an unexpected situation and there is a non-trivial time to work out what is happening and what to do. There have been many accidents caused by precisely this issue - you need time to acquire situational awareness and get it right.
Let us assume that the chamber provides all the protection and resources needed for the guardian to survive when 'awake'.
Seems odd that someone would design a chamber to keep them protected when awake, but neglect the rather critical safety of the cryogenic part of the system. If the cryogenic system is not considered 100% safe (or as close as physically possible) it's simply a waste of time.
Engineers don't design systems that are assumed to fail, the designed systems that are assumed to fail one way, but have backups and backups for the backups. That way the one in a thousand failure won't be a problem without a one in a billion combination fo failures.
If it's that important, it will be built to work and last even with problems.
Will the guardian survive under these conditions?
Baring unforeseen problems and employing your "handwavium" in the first place, why would they not survive ?
In "reality check" terms, they're dead, because physics and chemistry aren't going to let you do these things.
Edit: As far as I'm aware cryostasis can lead to tissue damage and I am expecting that there may be certain side-effects that are not directly influenced by slowing down chemical reactions.
"Slowing down" chemcial reaction is not what happens and that's part of the reason why you get "side effects". Either way if you've now turned off handwavium physics and expect the patient to survive, you are in for a nasty shock.
The scientific term for what you're talking about is Cryonics. While people have paid to be "frozen", the problem (especially with regard to reality check !) is that no one has been unfrozen. So-called cryonic institutes take money to freeze you and (supposedly) keep you stable, but they have no idea how to reverse the process.
Reality check : For "cryonicically frozen" read "frozen corpse" at the moment.
Should human bodies, as a hypothetical example, require radioactivity to function, the guardian would not survive since the life-preserving radioactive matter would decay.
I have no idea what you mean by this, but essentially this idea as you describe it is either this :
They designed it to work and wouldn't have used it if it did not do so reliably. Your handwavium physical process (whatever it may be) does all the stuff we don't know how to and works flawlessly because it's designed to.
... or this ...
We can't do it and we don't really have any reason to think it can be done successfully. Some con artist in your story convinced the originators of this madness to chuck a human into the "device" (= "fridge" :-) ), and skipped with the money in the certain knowledge the con artist would be long dead before anyone came back and said "it didn't work". And a government probably would fall for this because they fall for the most ridiculous cons all the time. But the stiff is dead.