My question is, do you think the fresh paint would stick to the single stage repaint if I hit it all with a light sanding or a scotch bright?
Will the paint stick? Yes. Will it look good? Kinda depends on your ability to lay down even coats. Will it last? Very hard to say.
You can definitely paint over existing single stage. Sand it with 400 grit, then shoot it with 2K primer, then shoot your paint. Adding the primer to your equation will only cost a little more, but will give you the substrate you need to have a nice finished product. You wanna knock the shine off the current paint, sand it down until it's a bit dull.
Wash the truck before you sand it. Get any contaminants off the current paint that you can.
Are you taking the trim off? If you can, you should. Yes, it's extra work but so is taping off all the trim on the truck.
Also, you do not NEED to go back with clear coat over single stage paint. It is already mixed in. You CAN clear coat it if you want, but it's not necessary.