The mouseclick location is 100% reliable. You're just Doing It Wrong(tm). :P

OK, seriously. Here's how you get it to work reliably:
  1. Make sure all clients are the exact same aspect ratio in the WoW in-game video resolution setting. Example: 800x600=1.3333 aspect ratio; 1280x1024=1.25AR; 1920x1200=1.6AR. Thest MUST match AR's for the click to scale correctly. Just do the math and pick the same AR modes on both main and clients. I have mine set to 800x600 on the slaves and 1400x1050 on my main.
  2. Make sure your in-game "UI Scale" setting is identical. If not, your UI elements (unit frames, buttons, etc.) will be different sizes/positions. Easiest way to do this is to go into \WoW\WTF\Config.wtf and change the "SET uiScale "0.75" variable to the same on all 5 accounts. (Note: need to have closed wow first for this to work.)
  3. Make sure your maximizer (in keyclone or octopus or maximizer.ini or wherever) sets your video resolutions to the same as you set up in step 1 above.
I have these three done and I can authoritatively say that you can click TINY spots in the same place with Octopus. Even as fine-grain as clicking between two letters in a macro when editing them.

Hope that helps.

Cheers,
Ugh