Unfortunately I can not provide you with a workaround to stop the loading the core.css file.
Could you check if it will be loaded multiple times when you use the OOB MOSS editor as well?
Regarding the editorcontentarea.css it is loaded in each RadEditor, because the content are of the control is an IFRAME and this is a css, which is related to the proper working of the control.
All the best,
the Telerik team
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