I have just discovered that there have been some issues in leaving comments which were, apparently, related to the fact that I was still using an outdated theme on the latest WordPress release. I'm not sure how long this problem has been around, my apologies for the trouble.
I'm temporarily using this new theme, and although I don't particularly like it, it's still better than a theme that doesn't let people leave comments at all.
It's still a bit messy, but it'll do for now.
More configuration guides for the Raspberry Pi are on my to-do list since quite some time by now. I'll put them up once I'll be more free.