The blog is on a domain, that comes with tons of email adresses. Doesn't it? So just ínvent a dedicated one that well may float. Change once appropriate.
Catching a form-post on the server doesn't take AWS (unless you only ask the Amazon evangelists). 50 lines of bash cgi do it for me.
The next step would probably be to use a mailing list program for the comments, allowing subscriptions / notifications.
Each microblog is a comment system – just reverse from what a blog wants.
The Fediverse really could help if just somebody would implement such a comment engine. But that engine has to be deployed decentral. On each blog.
I’m really not sure what you mean with that. The old blogs generation had a pingbacks system, but it’s really not suitable for commenting: that is usually not moderated by the target, it requires having set up your own blog, it’s really inconvenient for discussions when you have to keep jumping through (unless you use a federation tool but then it’s back to centralisation), and it can be awkward to reply through a blog post as the reply might not rise to that level of interest to your own subscribers, plus the subject at hand might not be one you aim to cover in your normal posting.
Imagine your blog is one, too and can receive likes and replies just like any other fediverse instance. Each comment is both on the commenters fediverse profile as well as under the blog article that it refers to.
What is your baseline and how is that simpler?
imo lambda/serverless can get overdone and too complicated but it works well for one-off services like this.
You want a custom domain anyway, don't you? The space won't be an issue then, is it?