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Because the function is going to be used for both actual and potential moves, it has to be able to handle both. For a potential move, #0 will still be occupied, and #1 must be empty, but for an actual move, #0 will be empty, and the move must not be a capture. So, depending on the value of #0, you want to confirm either that #1 is empty or that the move is not a capture. The portion of your code devoted to this looks like this:

and nor capture nor empty #0 empty #1

First, "nor empty #0 empty #1" returns true if #0 and #1 are both false. This value is used as the second value for the next nor. So, "nor capture nor empty #0 empty #1" returns true if #0 and #1 are not both false and it is not a capture. This does not seem to be what you want, since it will always require a false value for capture. But this matters only when #0 is empty, and it could throw off the evaluation of a potential move.

It would be better to use this:

and cond empty #0 not capture empty #1

This returns "not capture" if #0 is empty or "empty #1" if #0 is not empty. The and requires this value to be true for it to continue. Note that this test has to be done only once. So your code can look like this:

checkleap #0 #1 3 0 or checkleap #0 #1 2 2 and cond empty #0 not capture empty #1 or checkleap  #0 #1 2 1;