"They really like white and black bread."
Translation:Ili tre ŝatas blankan kaj nigran panon.
"Word order is pretty fluid" is true sometimes but not always. What this really means is that adjectives and nouns can be moved around a little and subject, verb, and object can be moved around a little. Even in these cases, there are better ways and worse ways to arrange your sentence.
Other words such as numbers, articles, and prepositions all have very specific positions which they must take in a sentence.
Related to your question are adverbs, which fall somewhere in the middle. They generally come before the word they modify - which is why it's "tre ŝatas" here.
The rest of your sentence is pretty good, but it has a different feel to me -- kind of like "They like white bread and black" or "they like bread, both black and white."
A lot of people like to say "languages evolve" in response to situations that they would never apply to a national language. I see this a lot.
If this were a course for English and someone asked whether they can say "unwhite" instead of "black", would an appropriate response be "languages evolve"?
English isn't an artificial language
This is exactly my point. You're treating Esperanto like an artificial language. It was a project 132 years ago. "Languages evolve" is usually something said about natural languages. I think what you mean is "projects can be tinkered with" - not "languages evolve."
I do assume that it should be “panojn”, considering that these are two different kinds of bread.
There's nothing anyone in this user forum can do. Next time this happens, take some screen shots and file a bug report.