Why is “un” in brackets? Is it optional, or is this a mistake?
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Here are some facts about “transformer en {something}” - or more generally about the “zero article” in this case.
French has this “thing” (let’s say “rule”) called zero article. It comes from the legacy of the language and makes the sentences prettier (to French ears at least). And this rule dictates when you shouldn’t put an article.
The case “transformer en” (same thing for “changer en”) follows this zero article rule when the object is simple - usually a single word. So basically you should be saying:
- Elle le transforma en insecte.
- L’alchimiste changea le bois en métal.
However if this object becomes a bit more complex, then the zero article doesn’t necessarily apply anymore.
- Elle le transforma en un terrible insecte.
- L’alchimiste changea le bois en un étrange métal très brillant.
Now back to your question 
:
As you can see, I wrote “necessarily” because this isn’t a golden rule. In fact, it’s not even a rule that French people learn at school. Instead it is something that becomes natural (by hearing it and repeating it), therefore the fact that the zero article “sounds prettier” (compared to having an article in your sentence) - because it doesn’t relate to logic but to customs.
TL;DR: The correct way is “le transforma en insecte”. The original translator probably hesitated and wrote “(un)” following a logical thought (while the zero article isn’t logic, unfortunately…).
Note: In some countryside of France (and maybe other French speaking places), they might omit the zero article thing. But yeah… countryside, hehe… 
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Thank you very much for your very detailed answer and my apologies for my slow response. I’ve just seen your answer now. (I thought I used to get notifications when someone responded, but I don’t seem to anymore.)
That sentence is correctly in Czech : “Už sem víc nechodí.”