I agree it would be helpful to have in the spec examples and explanations like the ones you give, and perhaps some explicit discussion of how the list rules interact with the tabstop rule.
I don’t think changing the tabstop rule would be a good idea; it is designed to ensure backwards compatibility with existing Markdown implementations that do a destructive tab expansion before parsing. It also makes good sense: if your editor tabstop is set to 4, you can use spaces or tabs as you like for indentation, and it won’t make any difference.