If you right-click on the menu (Manjaro) icon and select "Properties, you can change some settings. Also, under "settings" you should find "Menu editor" (menulibre). It allows you to make other changes like adding and removing launchers, but I don't think it allows you to change the guts of Whisker Menu itself.
I haven't tried it, but (after making a backup!) you can play with:
Edit: There's also
~/.config/menus/xfce-applications.menu which is (I think) what menulibre works with.
As for multiple columns, I have no idea. Might be worth looking at different menu plugins to the panel.