I've partially solved the first issue, although the command is slow in huge folders with many items (precisely where this command is most needed 
), and it doesn't seem to be due to enumeration since it's slow even at the beginning of a folder when only a few items are enumerated to find the target item's index. And at the end of the folder the speed doesn't seem materially worse
JumpItem↕ By=5 jump down 5 lines
JumpItem↕ By=5 NODESELECT SELECT same, but also select those lines and do not deselect existing selections
(-5 would be jump up)
see updated script here Jump Up/Down by a given number of items