Translation:She is full, so she eats very little.
"she's very full so she's eating very little" was rejected but I think it's fine
I wrote "She is full, so she is eating very little" and it was also rejected.
Should be "ate" instead of "eats" because "full" is a short-term condition that does not apply to a person in general (like a physical feature or character trait would).
"She is full so eats very little" is also perfectly acceptable as an English translation! The narrow range of acceptable translations is annoying to say the least!
These courses appear to be written by translator used to being paid by the word. Smoothness and rhythm be damned!
"得" is usually used for adverb of manner, e.g. He eats very quickly. "Very quickly" here modifies the verb "eats", i.e. "how does he eat?".
In Chinese, we usually use the double verbs when we want to use the adverb of manner, e.g. 他吃饭吃得很快。I don't know whether 他吃饭得很快。(using a single verb) is true or not.
"得" is often or usually necessary in Chinese, which is why it's optional to translate it as "very", since "very" is not necessary in English.
Just to clarify, 得 has no semantic relation to "very" here. It's a grammatical marker that indicates an adverbial modifier on 吃. I'm not entirely sure, but I believe in this case you could probably omit the 得 since 非常少 also can be interpreted as describing the object being eaten. However, if the sentence used 慢 (slow) instead of 少 (little), then 得 would definitely be necessary.
When you say "she eats very little" it sounds like something habitual, not something resulting from already being full. One more natural way to say this would be "She is full, so she's eating very little".
The English translation is not "natural" language. We would say, "She is full, so she is eating very little" or "She is full, so she is not eating much."
I don't know if the Chinese example is written to say that? I guess I expect to see a "再" used to indicate present action, similar to this: 她很饱，所以再吃的非常少. I don't know if that's grammatically correct in Chinese though.
I believe using any of the various present continuous markers in Chinese is always optional and only used when emphasizing that the action is ongoing. That's why they're not used here. The -ing in English is idiomatic in a way its Chinese equivalents is not, so like many other Chinese particles and grammatical words it's not translated literally in English.
When we use adjective, don't use "是" (to be). In affirmative/positive sentence, we use "很" as "to be", so "很" doesn't always mean "very". In negative sentence, "很" is unnecessary to use, except you want to use it as "very".