Trying to practice on these "already ruined" parts won't give any better results if you don't first remove the old botched paint and clean the parts. For example, if the botched paint job has a grainy, almost rough feel to it, no matter how perfectly you apply the new test paint job, most likely it will have a grainy, rough feel to it, as well, leading you to believe you aren't doing any better.
You ask these questions, and these grizzled veterans of the model-building hobby have given you all kinds of important advice and suggestions. It seems that you disregard their advice because it isn't what you want to hear. Most of these guys agree with each other, with a few variations. However, just about every model-builder has a technique they use that others don't use. They learned their techniques by practicing.
I've been building these things since 1963, and I have never achieved the perfect paint job. Probably never will. But, I have certainly backed up and started over, and at least did better the next time on some of my builds. I've had a couple that I stripped the paint off of at least three times! Buying products to strip paint, for me, is less expensive than chucking a kit in the bin and buying a new kit, just because I messed up the paint.