" ... the person who allowed the test version to be "customised" to look like the live version."
Not really a 'cretin'.
It would seem that this fellow just had a temporary lack of (essential) foresight.
Far too many times we tend to think of the end user as having the same type/sort/brand/flavour (if any at all) of common sense we have.
And that is a big (BIG) mistake which you usually end up paying for.
They don't, so you just have to play safe and make up for it in advance.
An option for the end user to change something/anything in the test version?
No. It should *not* have been there.
I mean, you don't allow third parties to meddle with anything in your test system, you have to have full control of it. Otherwise, how do you know what's going on?
Have a good week-end.