Thanks, everyone! This is a follow-up post where I'll answer some questions (answers in spoiler blocks, click to open) and provide some more information.
Btw, which is the original voice before the Ferrari – Black stops the car on Mission 6?
Those voices are pre-existing sound clips from The Simpsons Game, just at a higher pitch.
Why I can't enter any vehicles during mission 2 and 4?
Those missions (well, all of Mission 4 and part of Mission 2) are designed and intended to be played without driving a vehicle.
One thing I didn't understand was what am I supposed to "avoid" during the part where Bart had to collect the cola crate? I just kept failing the mission as I walked over to collect it.
The idea in that stage is that you avoid getting too close to the Kwik-E-Mart (the fail trigger covers the glassy area and extends to the parking spaces in front of it, and the idea is that you could get caught taking the cola by Apu, since the truck was delivering it there), but on reflection I can see that it would've been better to clarify in the mod somehow what "detection" meant. I'll make sure I do that.
I've become aware of a couple of things I'd like to improve/fix up (for example, the cola crate stage being a bit vague, and card quotes still showing), so soon enough I'll update the mod to improve these things and more. It'll be a soft update, though, so I won't bump the thread.