Hi,
I actually use CodeIgniter a lot, and have a lot of features that could (in the future) add to this platform you are looking to create. We use CodeIgniter for MSSQL, MySQL, Post Ingres SQL and sqllite, across shared hosting, cloud hosting and dedicated hosting and we have a CRM package that uses OOP PHP and CI to deliver Word Documents, Excel files, expert tags, appointment bookings, management reports, vehicle diagrams and a whole lot more. We also have one of the most comprehensive Auth modules in existance so if you would like to work with us on CI, we would love to show you how we do.
The blog module we can offer, would support tags, posts and login ability as well as using our templating engine (unless you have one). using a third party open-source content editor, admin back-end would contain edit, create etc functionality and if you like can serialize output to static files for a real performance boost without worrying about fail-over, replication and a lot of the features you mentioned in your original post. If you need login capability to one of our CRM systems please ask but also appreciate we cannot give login details to client "live" systems and will not give any code..
Oh and "Codeigniter HMVC"
Thanks for your time!