In isolation it's hard to distinguish. But if you find 30-50 reactors all over the globe that were all critical at the same time and all went silent at the same time, I think you'd have to think very hard about how that happened.
As for 1) current proposals (which continually fail to be implemented) are actually to concentrate the stuff and put it in geologically stable locations (c.f. Yucca Mountain). That would seem to be easier to find.
And 2) isn't really plausible. You can't select out and "convert" isotopes like that. You get the fission products you get, and your choice to get "more" out of them is really about how long you are willing to wait for them to decay.
For (1), we choose well hidden geological location, e.g. stuff that is about a mile deep. My guess is we don't have such an extensive knowledge of geologically stable locations one mile below surface.
For (2), I'd venture to say that this is based on what we know today. But then, nuclear reaction or coherent light or traveling to the moon were not exactly "plausible" in 1900 either.
What we know from physics is that there's still a lot of residual energy that could theoretically be extracted, to the point where these isotopes are naturally radioactive.
So to me, that means we are likely to one day figure out how to harness that residual energy.
As for 1) current proposals (which continually fail to be implemented) are actually to concentrate the stuff and put it in geologically stable locations (c.f. Yucca Mountain). That would seem to be easier to find.
And 2) isn't really plausible. You can't select out and "convert" isotopes like that. You get the fission products you get, and your choice to get "more" out of them is really about how long you are willing to wait for them to decay.