The latest sandbox build (2019.2.3812) now supports rowHeaderColumnsResizable property which lets you disable the row header column resizing:
dp.rowHeaderColumnsResizable = true;
When the resizing is disabled, the splitter width will be set to 1px.
You can apply custom styles to the splitter by overriding the styles for the "scheduler_default_columnheader_splitter" class. You can also generate a custom theme using the theme designer (https://themes.daypilot.org/) and manually adjust the output CSS file.