I think regular potatoes lightly boiled (I prefer soft for the turtles, potatoes are pretty hard), are fine, as a rare treat, since they too, contain lots of starch.
Frozen Vegetables are not as good as fresh ones, they have been frozen, so some (maybe most?) of the nutrition in there is dead, I'd personally go Fresh or none at all, but your choice. And boil them for sure, frozen vegetables are like steel to the turtles, they have strong jaws, but they aren't THAT strong. If my turtles have trouble chewing their pellets, then frozen is probably even harder.
I would say diced up pieces of Pears are best, with a big chunk, like any other large piece of food you drop in there, it gets ripped and torn up, and gets the water messy, also your turtles have to take more time and effort to get a good bite in. Not sure about the skin, but all the skin really is is fiber, nothing more, and turtles get plenty of fiber from vegetables and a bit from their pellets so I wouldn't put my vote on skins only. Your turtles probably wouldn't like to each just pear skins either, unless there's a bit of fruit on it,

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