Salve! Personally, I would be a bit more forgiving when it comes to the usage of "tempus" and "veri simile", but on the whole of course I agree with you. As to how it could be that the translation is as it is, I think there are two contributing factors:
First, while the translator might have a good passive understanding of Latin, and while he might be an excellent teacher for all that I know (at least with the support of a textbook and grammar), I think it is provable, from an analysis of the consistent types of errors, that his active knowledge of the language is rather patchy. To demonstrate this, let me give just one example of a particularly egregious recurring error, which has not already been mentioned in the reviews here, namely the faulty usage of the reflexive pronoun "se" (as well as the adjective "suus"). Seeing that English does not have this kind of reflexives, it is perhaps understandable if an English student occasionally mixes up "se" and "eum", but what is particularly noteworthy is that the translator also uses these reflexive words in the first and second person! Consider "noli se vexare" (p. xliv), "ubi se abdidistis?" (p. cxxxiii), "secum cogitavi", "se numeratis hobbitum abesse repperimus (both p. cxxxiv), "cum turba sua atque clamore araneas excitauistis" (p. clxxxi), "boni esto, se curate" (p. cxlix), and "cur tu se saepis, sicut latro in latibulo eius?" (p. cclxix). Obviously, this is not the result of carelessness; he simply has no clue how "se" is properly used.
The other contributing factor, I believe, is that the work most likely was rushed, in order to be published before the film comes out. Grammatical errors aside, there are lots of simple misspellings (such as transposed or missing letters etc.), which to me indicates that no one has taken the time to read through the book twice before publication, not even the translator. As you noted, the stronger part of this work is the poems, which perhaps is not surprising considering the translator's demonstrated interest in Latin poetry. Hence, it is plausible that he spent more time on the poems, and perhaps the work as a whole would have benefited if the same effort had gone into the rest of the book. (That said, there are problems with the poems as well; I'm thinking particularly of metrical errors in the quantitative verses: "triginta equi albi colle rubro maneant" is supposed to be a pentameter, forcing the first syllable of "equi" to be scanned long; "hic similis illo, disiunctus quisque habitabat", hexameter, thus long "-is" in "similis", even though it has to be singular which regularly has short final "i"; "necantur reges, montes caeduntur et urbes" has the same problem: first syllable of "necantur" is forced to be scanned long, even though the vowel is short; "sed pedibus tandem sic itur usque domum", pentameter, which means that the first syllable of "itur" has to be shortened.)