Basically, Regret to see is before the fact (it hasn't yet vanished), Stenka, and regret seeing is after the fact (it has vanished). In the case of your #1 and #2, Pascal is simply viewing the potential (future) disappearance (1) from a nearer future perspective and (2) from a farther future perspective.
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CalifJim -- though personally, I would have chosen "regret seeing" had I been the translator.CJ