"That was a very important scientific discovery."
Translation:Ese fue un descubrimiento científico muy importante.
I'm not so sure "eso" would refer to discovery. Seems like "eso" would refer to the specific thing that was discovered. E.g., penicillin or the theory of relativity. Since we don't know from the sentence what the discovery is, why wouldn't it be neuter? (Or do they assume the specific discovery would have been referenced earlier in the conversation, so they are looking for either ese OR esa? Hmmm... .)
I'm not a native speaker, but the way I see it is to first remove "muy" here, just to make it less confusing. Then we have "descubrimiento científico importante." Then this is simple Spanish word order where adjectives follow the noun. I believe you could switch the location of the words cientifico and importante if you wanted to, but then it just becomes a matter of which would be more clear to the listener.