There is considerable room for confusion on the Mac because the naming isn’t consistent or clear. In the prefences->shortcuts window the commands are all named by their path on the SketchUp menus. The strings to send to Sketchup.send_action() correspond (I think) to the Objective-C methods that activate each Tool in the Mac binary. I’ve verified that all of these work.
Menu path => send_action string
Draw/Arcs/Arc => “selectArc3PointTool:”
Draw/Arcs/2 Point Arc => “selectArcTool:”
Draw/Arcs/3 Point Arc => “select3PointFitArcTool:”
Draw/Arcs/Pie => “selectArcPieTool:”
On the Mac your current SU 2017 shortcuts are kept in
~/Library/Application Support/SketchUp 2017/SketchUp/Shortcuts.plist
The Shortcuts.plist file is xml with three entries for each shortcut: the letter key (string), the modifiers code integer), and the send_action (string). The modifiers code is a sum (or bitwise or) of flag values for the modifiers (the command key isn’t actually legal in user shortcuts, it is hard-wired to menu-defined ones and I am not sure what happens if caps lock is on):
0x010000 = 65536 = caps lock key
0x020000 = 131072 = shift key
0x040000 = 262144 = control key
0x080000 = 524288 = option/alt key
0x100000 = 1048576 = command key