I deleted my posts to avoid any confusions.

Now i better understand how kangaroo jumping works, i didn't even know that it is possible to find a publickey which is not in the space we are searching for.
That's why it finds the publickey when you just change Y.
I thought that when a tame kangaroo jumps out of space, a new tame and wild kangaroo would be set within the space to start jumping again.

As my suggestion, it would be the same if you just set the searchspace for from 2^(bit-1) > (2(bit)-1) to (2^(bit-1) + 2^(bit-2)) > (2(bit)-1).
You would have a 50% chance to find the key in sqr(2^(bit-2)).
In my tests it must have been luck to find the keys not in that 50%chance in less time than average.
The only benefit i see by pushing the space with the middle to Infinity point, would be if a X value match could be detected when a wildkangaroo starts in negative mirrored space. That would possible be a rarely case by the time it catches up to the positive space where all tame kangaroos would start.