force excerpts to always be the full post

That doesn’t even make sense.

I don’t know what you mean by “importing a bunch of posts from various other social media”, but when a theme outputs a post on its own page, it shows the full post (if it is paginated, it shows the current page). When a theme is outputting an archive page such as a category or the latest posts page, it has a choice of using the_excerpt() or the_content(). The excerpt will always be either a manually created summary or an auto-generated (strip HTML tags and count to excerpt_length number of words) shortening of the full post. Showing the full content can also result in a Read More link, because the author can put <!--more--> in there to indicate how much to show, but that only applies to the_content() and archive pages.

Most themes use the_excerpt() because it’s a better experience for the user, and you don’t run afoul of the search engine’s duplicate content penalties.

I know what excerpts are and why they exist, my problem is the fact that every post on the blog is somehow an excerpt despite me not using any <!–more–>’s in the post or having anything set for it to show up as an excerpt. All posts on the main page are just showing as excerpts automatically.

It’s not even the archive–it’s the main page itself. Every post can only be seen in full if viewed by itself for some reason.

And this is the only WordPress I’ve had that has this problem.

Just in some, while it doesn’t have the read more, it’ll still have a “…” as if there’s supposed to be and then nothing as you can see in the current theme (a default one) being used.

For some reason, all posts on blog are in excerpt mode despite not being set as such.

There is no such setting. Some themes might have options you can choose how to display lists, but it is not a WordPress option. It is controlled by the theme. As you’ve found, most themes show excerpts.