a) I want all users that register in the site (and take the āRegisteredā user type permissions to NOT be able to create a blog but only to comment on other usersā blogs.
Within CB Blogs > Parameters > Blogs set Create Access to whatever you like. There is no commenting in CB Blogs so that's entirely controlled by whatever extension you're using for commenting.
b) I want to setup K2 with a user permission that will have to be assigned by hand ONLY to the users that can create blogs and comment on other usersā blogs
Create a new view access level then set it as the create access (see my reply above).
If you're not using CB Blogs for this then please contact K2 for further support as we're not the developers of K2 and can not provide you details regarding its usage.