So I just (finally!) go the copy of 2001: A Space Odyssey Criterion Collection and I'm a little disappointed by the condition... The box is, well, it's been around we'll say, but there's nothing to be done about that.
More importantly, the discs themselves look as though someone poured lemonade on them and wiped them off with a rag saturated in more lemonade, and then put them in the sun.

I'm guessing this is early stage laser-rot? I don't see any dark regions, nor pockets of visible separation and no fractal like growths, just the 'lemonade marks' I tried wiping them off with a moist cloth, but no dice, the lemonade is in between the layers, I guess.

So... I haven't tried playing them yet, I'm not too keen to actually, but is there anything I can do to mitigate or stop the progression? Or has this train left the station...

Then bloody play them!

For laser rot to be visible to the naked eye, you're either way, way past "early stage", or it's not laser rot at all.

Laser rot in the usual sense is invisible to the eye. What you have is a ruined LD.

While this is a sad situation that you actually bought discs that were in poor condition your guessing is wrong.
You can't see rot on a disc that is not playing period.

If it looks like dried lemonade then wash it or wipe it off before you play.

I'm sure you have other discs that have inner sleeves to tell you how to clean discs, I'm sure you have cleaned CDs
so do the same with LDs and try not to get the label wet.

If it is the aluminum layer, there is no repair. I have a PDO disc that looks awful, but only the audio is affected.

Maybe someone tried to "clean" it with a non-PMMA compatible cleaner? If so, you could probably polish it off with Novus.

The Criterion 2001 CAV is not a known rotter. Regardless, the seller should just accept a return for selling a disc set in such poor condition.

Request a return and patiently look for another copy in better condition. This is not a sought after title so prices are reasonable for it.

And as everyone has already mentioned, “laser rot” is not something you can physically see with your eyes (at least 99% of the time). You play the disc and see if there are excessive drop outs in the video.

Would you mind taking a quick photo?

My understanding is that when laser rot happened it happened within 3 months of a disc pressing so the odds of any of your discs getting it now are zero.... And it was certain plants like Sony caused the problem. I played my criterion CAV 2001 not so long ago and it was great. My only gripe or annoyance is that the colour palate is completely different to the version we get today either on blu-ray or 4k.

That’s not really true…what you’re saying about rot. It was true when the only factors were the pressing plant, transporting it to the store you bought it from, and maintaining it in your house. However I can make a perfect LD start rotting right now if I wanted to…just leave it outside for a year in Florida or submerge it in sewage (same thing). LD is now a dumpster diver’s eBay hobby and lots of LDs are out there not being taken care of for decades on end.

I live in a temperate climate so those problems don't apply to me. Storing and looking after those silver platters is no problem here.

As long as you can keep the damp out you’ll be fine. My collection has been in Michigan for 30 years and nothing has ever rotted on me during that time

I live in a buffer zone so it never gets too hot (no humidity) during the summer or cold and extreme during the winter. I think the last time we had an extreme Summer and even then it wasn't humid was 1976 and you have to go back to 1962/63 for an extreme winter... I call it the laserdisc friendly zone...