I wouldn't recommend mixing in the paint cup full stop. You risk damage to the inner surface of the cup (the needle too) by possibly scratching it with the ferrule on the brush for one thing, for another and more importantly, you will not get a thorough mixing of paint and thinners, I've seen many videos where people decant paint then thinners and backflush to try and mix, likewise, this won't work either.I mix in little plastic medicine dispensing pots, (chemist shop, ask at the counter, they're pennies and if you're a regular they might even give you a few for nothing).
This way you can see when the paint is mixed AND when the consistency is the skimmed milk translucency you're trying to achieve.
If you mix in a pot as you'd do in the cup, (especially so enamels), you'll see what I mean about not mixing thoroughly. The only time I'd decant into the cup is a paint that requires no thinning.
On the subject of retarder, it's a chemical which allows the dying process of acrylic paint, (used by silkscreen printers too), it's purpose is solely this and not thinning and it allows you to get a wet coat down before the paint dries.