Inexperienced users really, truly need WYSIWYG; that's why we have so many WYSIWYG options.
In this case, even experienced users demonstrably need WYSIWYG, given what you noted about matching mood text overriding selecting a different mood icon.
I feel that having a checkbox to be able to select a mood icon for custom or conflicting text is adding an extra step to simply doing it.
The more I think about it, the more I think that what I'd really want is to type in the text exactly the same way we do for tags as per the original suggestion (and definitely including the part where you type a space to get the entire list to select from), and then where the appropriate icon currently displays, the entered/selected text shows up beside it, and then if you want to you can click the mood icon to change it, or click the highlighted/outlined/otherwise-obvious space where it isn't to select one for iconless custom text. Simple and unobtrusive, but does everything you need.
no subject
The more I think about it, the more I think that what I'd really want is to type in the text exactly the same way we do for tags as per the original suggestion (and definitely including the part where you type a space to get the entire list to select from), and then where the appropriate icon currently displays, the entered/selected text shows up beside it, and then if you want to you can click the mood icon to change it, or click the highlighted/outlined/otherwise-obvious space where it isn't to select one for iconless custom text. Simple and unobtrusive, but does everything you need.