I've been installing distro after distro, trying to find something I'm satisfied with. I used Fedora, for a while. Got tired of Gnome. Installed KDE Plasma. Like it, but was unsatisfied for one reason or another. I tried Manjaro. Didn't feel like learning my way around an Arch distro at the moment. I got over my parochial dislike of Debian based distros and tried Ubuntu and Mint. Ubuntu, Gnome again. Mint, I had a couple small issues that bugged me. So, finally, I've settled on KDE Neon. For now.

Well, I installed VirtualBox via apt install. So, I create a new virtual machine. In settings, I attempt to map the ISO I'm going to use and when I click 'Choose a Disk File' it opens the Dolphin window to choose the ISO, but aside from the X to close the window, I am unable to select anything in the window. The window is apparently dead.

How do I troubleshoot this past reinstalling VirtualBox?

Have you lost track of what your host distro is?
It sounds like you could install Vagrant as a VM box manager.

Actually I find that VirtualBox is a bit flakey. If I try to install something with a reasonably decent screen size, it always seems to revert itself to a 4:3 aspect ratio.

Why not just try out booting from a USB key?

Can you add a screenshot of this window to your question?

The left sidebar of your Dolphin window should have a Places section. If the ISO file is in your home directory, select Home. If not, you can select root and navigate down to the ISO file.

Another section in the left sidebar is Search. You could use that for searching the ISO file.

I am not certain, but you should also have a field where you can enter the absolute pathname of that ISO file, i.e. the name that starts with slash "/".

Attached the screenshot to this reply. Won't let me do it in an edit.

The Places column is just like the rest of the window. Dead. The only things that work are the Close, Maximize, and Minimize buttons.

Attached Thumbnails

 

"VirtualBox ... Oracle VM VirtualBox Extension Pack?"

Edit/add: I also just thought of right clicking on an ISO to open it with virtualbox or its media manager app?

Bit strange, are you able to test using another window manager to eliminate the possibility of bugs?

After miserable experiences with VirtualBox , I found that the surest way to install is to download the latest from their website, choose matched Deb & Extension Pack. For example, I use MX 19, so it will be Debian 10.

Mine is fine from Sid.