Thanks for the details, it made my job much easier.
I have managed to find a decent solution for you. First, the custom field selectors support empty values. (link to the documentation) This is basically used to add an empty choice to the list, which the plugin ignores. I've added one extra option to the service, key market, expertise fields (“Select …” option). You can check that on the back-end of the plugin.
This solves the problem you described, as if the user selects location+expertise then the others are ignored. However he can still select the other ones. To resolve that problem, I have put together a custom script, which you should put to the footer.php file in your theme directory, before the closing body HTML tag:
This will make sure that if the user selects one from either service, key market or expertise – then the other two fields are reset to defaults (empty).Best,
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