Messages broken in High Sierra

After reading more about reinstalling macOS and realizing that all of my files and (most) of my settings would stay intact, I decided to make a Time Machine backup and then just go for the reinstall. However, to my surprise, not even a reinstall fixed the problem! The same behavior as before continues to persist. The only option that I can think of is a full reinstall of the system from a clean disk, which I am trying to avoid at all costs.

Thank you for your continued help with this problem. It seems like something that just won't get fixed! Haha.

(EDIT) Another issue that I don't recall having mentioned before: when I am in Messages and someone else sends me a file (for example, an image) I cannot double-click the image to Quick Look at the file. Right-clicking and selecting Quick Look also doesn't work. Not sure if that is related or just happens to be another problem. Quick Look DOES work in Finder and other programs.

Another follow up: since I can receive files just fine from other people, I had someone send me a file so that I could watch the Activity Monitor and see if IMTransferAgent ever runs...and it does! I tried waiting for IMTransferAgent to start up from receive a message and then quickly sending a message before the process stops, but (probably as expected) that did not work. I am at a loss for how to fix this.

Perhaps part of the problem is that the IMTransferAgent service isn't running? I can't find out how to manually run this service. It may no longer be used in macOS 10.13?

I did that (wiping, reinstalling).... didn't fix anything.

Unfortunately this did not resolve the issue either. No change in behavior.