Just wondering (I'm a bit new and might have gotten it wrong) but why is the low clearance level version Safe-Class, but the four (4) up clearance level version Keter-class?
Is this because the temptation itself makes containment very hard? Or is it due to the time-line altering effects? Now that I think about it, it probably is, but I'm still just not sure.
The former. They want to make the anomaly seem like something dull and harmless to those without clearance. They're not totally sure they trust themselves.
Also, surely a safe-guard could be using SCP-3997-1 subjects as immediate containment personnel, with an outer perimeter under direct Foundation non-anomolus control?
Well, the problem is that they don't know exactly how the rose garden is activated. All they know is that every time they've found it, it's been activated. The only solution is not to go looking for it. It could have a cognitohazardous effect compelling people to enter it; or maybe nobody who's actually physically there can resist the temptation. Or maybe they don't need to enter it at all, and just knowing where it is activates it.
Either way, they've eventually decided that the only thing to do is leave it be, short of flooding the entirety of Gloucestershire and probably destroying their secrecy. Though I have a headcanon that the Foundation will eventually do that.
As for the -1 subjects; they aren't immune from activating it multiple times. They could just as easily rewrite reality as the others.
Once again, I'm new so I dont know if anything I've said has merit or should even be said.
They were perfectly sound questions, don't worry :).