I can assure you what you're experiencing is not any issue with the editor, rather it's the game itself. I can be reasonably certain you're on version 2.12 or beyond because everything you posted about is something that has been known to happen in these updates for quite some time now.
1. For instance, the Red Bull X2010 JP Flag Color was removed after 2.12 and attempting to use it will cause everything you just posted about to happen. (Though, I must confess, I did not know that it froze upon receiving the car) Hence why I said this mid way through the tutorial:
Red Bull Charity C gives the Red Bull X2010 JP Flag Color, which was removed after 2.12, so you will not be able to use it if you’re on 2.12 and above. (the function does work though)
However, if you downgrade back to 2.11 or below, the car will work then without any issues.
2. Regarding the XP issue, I have seen some posts saying roughly the same thing you said. I am not 100% certain, but I think after 2.12, they did something to make it where your XP will more or less go back to what it was before editing it in the save editor. Which means what you're experiencing is probably normal if you're on 2.12 and beyond like I think you are. You probably won't have this happen on 2.11 or below.
Though it's worth noting GT5 isn't entirely flawless either because I made it to level 40 for both a-spec and b-spec legitimately without save editing, in fact I think I did it before save editing was even a thing in GT5, and despite not being on 2.12 and beyond, I still had my XP go as far back as level 30 after a race even though I had never edited it and was on 2.11, the update before the XP revert was added.
So point is, none of this is the editors fault, rather it's the game itself. I have to ask, what update are you on anyways?