I’m trying to select and delete a tiny bit of a wall, however, when I try and select it, not only does the desired bit not highlight in blue, but it selects a completely irrelevant element. I’ve tried everything, the shape is definitely fully enclosed, any advice would help.
I see you have reversed faces that need to be fixed, you’ve made a group of some of the geometry on the wall with the garage door, and you are using layers incorrectly.
I made a copy of the group. The rest of the walls are ungrouped. If what you are trying to select and delete is inside the group, you need to open the group for editing.
Explode the group you made of the wall geometry, select the wall geometry in that area and run Intersect Faces>With Selected. Then you should be able to erase what you want to get rid of.
By the way, if you’d purge the unused stuff from your model you could reduce the file size by about 58%.
After I exploded the group, all of the geometry of the walls is ungrouped. Intersecting faces or tracing the profile edges around that part of the window should join the faces so you can erase the unwanted edges.