SlickEdit Pro 2020 (v25.0.1.0 64-bit)
Go to the top of a file.
Shift select+Down to select several lines.
Ctrl+Shift+| to add multiple cursors.
#1: Press END: Now the top line is deselected, but the other lines remain selected.
#2: Also, pressing END again toggles the last cursor to line up with the first one, or be at the end of its line.
#2b: Interesting ... actually pressing end again toggles the 1st and last cursors to line up with the longest line in the selection, or stay at the end of their respective lines.
Once you clear the selection (Say, by pressing left or right arrows), END toggles the position of all the cursors.
#3: Set multiple cursors as above
Press HOME: Properly deselects, but keeps multiple cursors.
Shift+Right a few times: Selects a few characters on each line
END: First and last line cursors toggle positions as described above.