I'm wondering if there's a keyboard-only alternative to the ALT - click & drag to perform column-type selection in the script editor... I'm used to np++'s ALT-SHIFT-UP and ALT-SHIFT-DOWN to do it, but can't find it in RStudio.
Yep. Press Alt+Shift+K to bring up the keyboard shortcut list for your platform (you didn't mention which OS you're on) and look for Add Cursor Above Current Cursor.
On MacOS, the keyboard alternative is Shift+Option+Up to create a new column cursor on the previous line (and Down for the next line, of course).