After some thinking and experimenting I've decided that stencils and actual painting can work out better than decals. Decals are really cool if you want to put an image on the shell but I just want to create something of my own

On clear coating:  I've shot clear over white decal paper before (usually with Krylon Crystal Clear, flat) without any issues.  I like to apply my decals to the base coat.  Some people advocate for brushing on some clear where you wish to place the decal, but then you risk trapped solvents creating bubbles.  If using water-slide decals, let it dry twice as long as recommended, just to be safe.  Once your decals are in place, apply multiple coats of clear, following the instructions on the can.  I have trouble getting some decals, especially water slide decals, to stay adhered to matte finishes.

One problem with decal paper is that you can't really use the transparent paper on black, or other dark colors.  The decal either won't show up at all, or the colors will be muted.  This sucks for when you want to do text or something else that's too finely detailed to cut around.  I'm starting to think that laser cut stencils are the way to go....

Some web research brings me to the conclusion that a clear coat should be used indeed. You'd want to use it even for solid paint job without any decals and images. My two concerns are - (1) will it whiten the paint / image? and (2) can it damage the paint / image? I understand that if you apply too much of the stuff then it's probably a yes for both, I wonder how forgiving are these clear coats?

While we're on the topic, I have a follow-up question, since I've got a couple of decal-related project ideas I've been kicking around for a while.
How does one make sure that decals and stickers and the like stay on for good? Should some kind of clear coat be used or what?

You can.
There are videos out there and i also have sheet's of the paper here. There are a lil' bit thicker, but not to much.
But still, it ain't so easy, and of course they look best if you make it on bright colors and/or best white.

and you can shove the decal paper in the printer or what?

at hobby stores you will find sheets of decal paper suited for home printers.  you can print whatever you'd like, trim it, apply, and clean.  pretty straightforward when you read the instructions.

there might be two kinds, one with a template you use to print out on pre-cut parts, and full sheets.  get the kind that is a full sheet of decal paper.  should come in a pack of 5, 10, 15, 25 sheets or something like this.

Greetings everybody,

There's a number of FAQs / tutorials about how to paint your gameboy in solid color or multiple colors. It seems to be pretty straightforward. What I can't find is how do I transfer an image on a gameboy shell? Like it's done in the picture attached. Thanks!