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In short, while it is somewhat possible it's complicated enough that I wouldn't recommend it unless there's a really good reason (i.e. someone is paying you per hour to do this).

WordPress does not really create multiple sites in a multisite installation. It has a single installation and uses different sets of database tables to make it look like there are multiple sites. Information about these tables and how they correlate to individual "sites" are stored in the actual (main) site's database. That's why WordPress now uses the terminology blog network instead of "multisite" and refers to each individual "site" as a blog.

What this means is that you cannot extract a single blog from the network since its data is entangled with that of the blog network (main) site which is in its turn entangled with all the other blogs. To make matters worse, WordPress holds theme and plugin configuration parameters in both the blog network site and the individual blogs. Since this data is serialized PHP data it is very complicated to merge them. For the same reason you can't directly convert from a blog network to a single site installation, dropping other blogs in the process.

What you can do is restore the entire blog network locally, e.g. using MAMP, XAMPP or WAMPserver as your local server environment, then follow these complicated instructions. I would call this process the diametrically opposite of user-friendly but at least it clearly demonstrates why such a feature is not implemented in an automated fashion...

Alternatively, you can use WordPress export and import features to export the content of a blog and import it into a new, single site installation. The obvious problem with that approach is that everything that's not a WordPress post needs rebuilding from scratch.

The issue you have is not with Akeeba Backup. Unfortunately I cannot help you with it. It looks like you have a really old multisite installation dating back to when there were two separate WordPress releases, the regular and the multisite. That was before my time with WordPress; we were a 100% Joomla shop back then.

I think you're better off asking these questions at the WordPress forum or a local Facebook group. There are people who have been around that long and possibly know how to help you with that.

Another approach is doing what I described and then doing two things. First move the files into the wp-content/uploads folder. Then do a mass replacement in the database to change /files/ with /wp-content/uploads/. However, you need a tool which won't bork your database's serialized data. We have been working on a plugin which does exactly that. If you're interested let me know, I'll send you an installable build. FWIW it's the same data replacement engine powering Akeeba Backup's restoration script (ANGIE) since December 2018 so it's actually been battle tested already :)