Install preloaded map on computer

No, there is no option to install the preloaded Edge maps on a computer.
But if you want to use these maps with Basecamp without connecting your Edge to the computer - there is a way:

Take a memory card, create a directory \Garmin and copy the desired maps into that folder. If your computer has memory card slots plug your card in and let it there. If no slots available use a card reader.

If you are good with computers you can also create a "virtual SD card" on your computer.

Have you tried to copy the maps? Copying appears not to work (BaseCamp says there's an error with the map file). I believe these maps are locked to the device.

I just did a test and copied the America Central map - gmapprom3.img to a SD card (it is the smallest img-file on my European 1030+) and it works perfectly. See it and can work with it in Basecamp - but have to admit: I'm still on Basecamp v 4.6.2.

I refuse to update because I've heard that Mapinstall does not work properly with newer versions.
Is it true?

I tried it with the latest version of BaseCamp.

One can get free maps that are also based on Openstreetmap and those work. 

Did another test on my WIN10 laptop with Basecamp v 4.7.3 (newest version) and that did work, too!

And I found another strange thing: IMG need not to be saved on a SD card but it also works with a simple FAT32 formatted USB stick.

So, you can't just copy gmapprom.img (North America, for me.

There's another file that is needed for it to work.

I copied the North America and Central America IMG files to my OPTIMA 16GB USB stick - folder \Garmin.
No other files like unlock or gma!

North America IMG has a size of 3,860,201,472 bytes.
The Header of the file looks like that:

The USB stick is a very old 16 GB stick called OPTIMA.

No Edge device is connected to the laptop!

Basecamp lets my chose the North America map and work with it:

What you describe is how it's supposed to work (I'm very familiar with it). 

I'm using a virtual disk. I haven't had any issues using this (I've used it for years on multiple computers).

sourceforge.net/.../

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There's other weird stuff going on.

I sometimes get the error and sometimes not.

BaseCamp also caches the map tiles, which does other funny stuff.

You can clear the cache for the current map by hitting Ctrl-G (twice).

It "works" now but some of the tiles don't show up.

Restarted BaseCamp and the tiles are now showing up.

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It's possible the errors I was getting was because the cache was trying to refresh the tiles from the device rather than from the copy of the file.

...and... it is always amazing how often someone asks a question that obviously no longer interests him...