Generally, yes, but it's a bit of a case-for-case basis when you can make exceptions. It's quite possible to say jag håller med dig väldigt mycket, whereas you can't say du ser ut väldigt hemsk. And do note that it's only for qualifiers, so a phrase like du ser ut att vara väldigt hemsk is correct.
Qualifiers are generally adjectives and adverbs. In your suggestion, "som" would introduce a prepositional phrase, a prepositional phrase with a noun, not an adjective: "like [someone/something]."
Here is a summary of an excellent explanation online: https://blogs.transparent.com/swedish/looking-good-in-swedish-att-se-ut/
When you’re saying something looks “like a dog” (i.e., “like” + noun), the formula is se + ut + som + [noun].
When you’re saying something looks “dark” (i.e., adjective only), the formula is se + [adjective] + ut.
Pattern 1: ser . . . ut with prepositional phrase
Det där molnet ser ut som en hund. That cloud looks like a dog.
Ser det där molnet ut som en hund? Does that cloud look like a dog?
Pattern 2: ser . . . ut with qualifier (adjective/adverb)
Det där molnet ser mörkt ut. That cloud looks dark.
Ser det där molnet mörkt ut? Does that cloud look dark?
another tip: Moln “cloud” is of neuter gender, so the adjective mörk “dark” gets a -t (mörkt). Hunden ser glad ut (not glatt). The dog looks happy.
Probably not. I searched all three books and I can't find that line in any of them - only in the movie. :)
As a beside, there are actually two Swedish LotR translations, because the first one - by Åke Ohlmarks in 1960 - was absolutely horrible. Not that the language is bad, it would actually be a quite decent tale if it was his originally - but as a translation, it sucks. Ohlmarks completely ignored all of Tolkien's suggestions, and remember: Tolkien was quite adept at several Nordic languages. In fact, The Silmarillion was only published on the condition that Ohlmarks not be allowed near its translation. So when the movies finally came, a new translation was made, by Erik Andersson (the prose) and Lotta Olsson (the poetry), which is by all accounts much better (although I haven't read it and cannot vouch for it myself).