When you delete all blog categories the blog entries disappear. Is it not more easy create a "Uncategorized" category similar to Wordpress? Easy, intuitive... You can't delete this category and only is visible if there is a uncategorized entry.
maybe a 'Bulk Processing of Articles" option in ACP -> Community -> Blog would help. Like there is in ACP -> Community -> Threads & Posts
for the other part,
user permissions for categories within the blog for articles already exist: ACP -> Community -> list Categories then edit the category you want. at the bottom allow a usergroup you create for this.
Then you could set up a usergroup with permissions used for the blog category and only leave Can Read Blog Articles checked, that way people who visit Uncategorized" category can only read and not delete/edit/or post.