OK, I finally finished the game, but I didn't get the DeSoto Dominator achievement, so I spent the whole day working on completing the achievement, and this is what I found out!:

1. In episode 5, when I checked the poster of decals on the wall next to the DeSoto, I noticed that there was a whole row of decals that were missing! I deducted that they were from Episode 1, so I went back to episode 1 and completed all of the 'tasks'.

2. I also noticed that the very last decal on each row was gold, except for the bagpipe for me, so I had to go back to that episode and try to hit as many bagpipes as I could to get it to turn gold.

3. Since this is a point and click game, I was using my mouse and keyboard on the mini games, but I found it easier to do these mini games using a game controller, especially for the episode 1 mini game I believe it was, because not only could you use the left stick to move right or left, you could also speed up or slow down by also pressing the stick forward or backwards, to help with hitting the items.

4. I thought the following rewards were only available on the console versions of this game, but if you complete all of the challenges for each mini game for each episode, you get the following rewards:

Episode 1: Turbo Charger
Episode 2: Dashboard Hula Girl
Episode 3: Flame paint job for DeSoto
Episode 4: None/NA
Episode 5: Demonic Grill Teeth
Super Upgrade: Police Lights on the DeSoto (if all others collected)

5. I did notice that for each episode's mini game, you would get the gold decal for like the third level challenge, but there would be just one more level challenge beyond that. For example, I seem to recall that I got the gold Elmer after hitting 25 Elmers in 60 seconds, but there was another challenge after that which was hitting 30 Elmers in 60 seconds. I'm not sure if beating that 'extra' level challenge is what gets you the rewards mentioned in 4. above!

6. Don't forget that on the screen where you can select "Continue Episode x", next to that is a "Chapter Select" where you can more granularly go to a part of the episode (If you have already completed it). I've found that the very last chapter in each episode will keep you from being able to go back to the mini games, so the second to the last chapter might be the best place to start at!

Anyways, hopefully these tips will help you go back and finally complete this achievement! (All of this info is as of version 1.0.5!)