Is there a universally recognized database or a "single source of truth" for music scores?
No. To find (or, more accurately, get as close as you can to) the definitive form of a composition, you have to do a bit of legwork, figuratively speaking. In some cases, much ink has been spilled by musicologists on various instances of this question.
The best way to be sure of the "correct" musical content is to have a published edition prepared under the composer's supervision or at least a carefully copied manuscript, either in the composer's hand or with the composer's corrections -- these are often prepared in connection with an engraved edition, but before music printing became widespread they were also frequently prepared as presentation copies.
Another very useful kind of source is performance materials. Parts give insight into the numbers of performers as well as the musical text, because any copying errors are generally caught and corrected during rehearsals. A composer's performing score, however, may be less legible than a fair copy (and in baroque works the continuo part is often "unfigured" in the score, where as one of the continuo parts will often be "figured," meaning that the numbers describing how the harmony relates to the bass note are present).
I put "correct" in quotes earlier because composers often change their minds -- or perhaps do not remember certain minor details -- so it is not uncommon that two manuscripts from the composer's hand, written at different times, might differ. Therefore, if you find an edition that disagrees with a composer's manuscript, it is not necessarily an error. The edition might have been prepared from a different manuscript, also from the composer's hand.
Therefore, your next question is very much on point:
Additionally, what strategies do musicians typically employ to ensure they are using a correct and authoritative version of a piece?
First and foremost, buy your editions from an established professional publisher instead of downloading them from the internet.
Second, if you want to be thorough, buy or at least consult every such edition you can get your hands on. Where they disagree, find out why. For music that is out of copyright, publishers add value by including a scholarly commentary on the decisions they've made and the sources they've used. Find those sources and see whether you agree with their choices.
Finally, if you don't have access to scholarly editions, you can still look for sources and do your own scholarship. The best places to start are https://imslp.org (look for manuscripts and first editions) and https://rism.info (they don't host any sources but they tell you where you can find them, and in many cases they link to images that are available online).
Since Sibelius died less than 70 years ago, his scores are still copyright protected in many countries, but not all. His publisher is Wilhelm Hansen. If you can't find the first edition for sale, you can also try your nearest music library.