OK, i see several mistakes here. First, just call the emulator Retroarch, this will give you some extra options, namely the "Associated Platform" tab for it is different and is also somewhat pre-populated. In your first image you have the load core command in the command-line section, that is not needed if the emulator is named Retroarch like i said, and it also means that your second image is different and lacking options, also you have partial core names there, that is likely completely breaking the process.
So, with that said, delete that emulator entry, and add a new one, this time call it Retroarch, point to the exe and dont change any other options or add any commands.
Once that is done go to the "Associated Platforms" tab, you will now see it looks different to your image, as it is already pre-populated with the most common systems and the our "recommended" cores for them (both system name and the core used can be freely changed).
As you can see GBA is a pre-populated system and is set to use the vba_next core, so to change that to mgba core simply double click the core that is listed, this will open a dropdown where you can pick from a list of cores (this list is actually the cores you actually have downloaded as we scan the core folder, so if a core you downloaded isnt showing restarting Launchbox should cause it to populate).
OK out of the window and games should now load, just repeat the "Associated Platform" tab part for any other systems using Retroarch. I should also note the system names in that tab, need to exactly match what you called the platform when imported. For example if you called the platform "SNES" then Retroarch wont load the games as it has that system listed as "Super Nintendo Entertainment System", so you would need to edit that name by double clicking the current entry and re-naming it to match.
Hope that gets you up and running.