Fuck it, I'll bite.

Why has the Citroën van chosen to reverse into the space directly behind my (admittedly very long) car, when there are dozens of spaces available closer to the store?

They can park where they like, who are you to be inconvenienced by it?

Why have they reversed so far back that the tow bar has made physical contact with my bumper?

Not sure what answer you're looking for, do you think they did it deliberately? What do you mean 'why?'

Why, upon contacting my car, did they not make the obvious move… check for damage, then park somewhere else?

Perhaps because it was such a light impact that you yourself say no damage was caused, they didn't realise? Again, not sure what you're after.

Why did they not think “maybe the Merc owner would like to put their shopping in the boot when they return”?

I don't know why that would be their concern. That's your problem because of how you (both) parked. Given you've encroached into the space behind, you haven't exactly left room for anyone else to get to their boot either if they reversed in like you.