In scenario (2), it's difficult to imagine any scenario in which the player needing snookers would benefit from refusing to pot a colour if one is available. Potting a red when already needing a snooker, but no colour after it, means giving away a further seven points, thus increasing the number of snookers required by 1 or 2. Since the colour would be returned to the table after potting, it's not like they would have reduced the number of balls on the table either.
It's not against the rules, though. Still, the only scenario in which they would refuse a pot is when such a pot is essentially impossible, or at least much harder than laying a snooker. More often than not, the scenario also arises when the red was potted accidentally (or, perhaps, when the player played a terrible positional shot). But you wouldn't pot a lone red when needing snookers, or if missing the next colour would leave you needing snookers, by choice.