How do I remove someone from Asana without breaking all the rules I have created!?

That’s tough.

This is why I advise against adding rules to templates, and if possible prevent rules that are bound to people directly.

A good alternative (if you have Enterprise) would be to use a bundle, as it will allow you to centrally edit rules for a group of projects.

What was the “otherwise if” condition exactly? If it was to check the assignee I might be able to provide you with another workaround.