How to split the paragraph on Enter instead of showing the content completion list?
To obtain this behaviour, edit your Document Type and in the Author tab, Actions tab, add your own split action. This action must have the Enter shortcut key associated and must trigger your own custom operation which handles the split.
So, when you press Enter, your Java operation is invoked and it will be your responsibility to split the paragraph using the current API (probably creating a document fragment from the caret offset to the end of the paragraph, removing the content and then inserting the created fragment after the paragraph).
This solution has as a drawback. Oxygen XML Editor hides the content completion window when you press Enter. If you want to show allowed child elements at that certain offset, implement your own content proposals window using the ro.sync.ecss.extensions.api.AuthorSchemaManager API to use information from the associated schema.