"When I fight multiple mobs, right-clicking the second mob to attack it is sometimes interpreted as trying to loot the first; not what I want."

You could select a mob to attack by left-clicking and then start auto-attacking by an icon on your castbar, or use the tab key to switch from mob to mob instead of clicking. Alternately, and this is the way I do it, play with enemy health bars enabled. Hit the V or shift-V keys to toggle them on and off. It's easy to see when you're hovering over them because the health bar lights up, so you only click on what you want to click on.

All of those options have a problem with a lot of potential targets - using the Tab key might require cycling through them until you get the target you want, and if there are a lot of mobs on the screen health bars and mobs might get pushed around the screen too quickly to be useful - but nothing's perfect, and they're the only ways I can think of off the top of my head.

I also have this problem. One option that I have used from time to time is to go to the standard UI and turn the option on to automatically loot the corpse. Since many mobs only drop an item or two (or none at all) this makes the mob quickly go dead and you don't have to worry about clicking it by accident again. The only down side to this is that you need bag space because you will pick up a lot of trash.

@Cyrus

"Alternately, and this is the way I do it, play with enemy health bars enabled. Hit the V or shift-V keys to toggle them on and off."

It's just V to toggle them on and off. Shift+V toggles the visible health bars of friendly targets.