It is not possible to set the selected element, but it would be useful if a way was added. You can still get the selected element with tpt.selectedl or tpt.selectedr
The element name for none is "none", "NONE", or 0. An empty string will not work if you want to set the type or ctype of something
"None" is just the pseudo-element for nothing. Changing the type of an element to "0" or "NONE" will delete it, changing the active cursor to it will make it erase particles.