Rashi in his commentary on I Kings states that the apparent contradiction is resolved if one understands that the additional 30 were from the Chirom's 120 since Chirom’s servants were in Ophir with his servants:
It is written in Divrei Hayomim that [the ships brought] from Ophir 450 [talents], however it does not include the 120 talents of Chirom. We can resolve the contradictions that the extra 30 were from the 120 of Chirom, because Chirom’s servants were in Ophir with his servants.
Additionally, commentary by Radak on I Kings highlights this alleged contradiction and states that in the Book of Chronicles it says four hundred and fifty gold coins, but in the I Kings it says four hundred and twenty and that's because the thirty were spent on road expenses:
ארבע מאות ועשרים ככר. ובדברי הימים אומר ארבע מאות וחמשים ככר זהב כי השלשים הוציאו בהוצאות הדרך:
As far as your comment on commentaries goes, I think it's a bit of a misconception. Commentaries don't necessarily try to insert additional meaning, but instead try to exegete and provide clarity on a particular text or to show answers to questions that people commonly have. Some features that commentaries often include are regarding resolution of alleged contradictions by drawing from multiple texts as a totality, trying to highlight important and interesting facts from the text that often get missed by many and have implications in certain contexts, and much more. I don't think that, for example, Rashi is adding any new meaning or text here, G-d forbid, but is instead is highlighting what is often implicit within the context of the Torah and the Jewish tradition. In the long run, I think that a good commentary can only be of help and enrich your understanding, and Rashi's and Radak's commentaries have a high reputation in Jewish circles so it wouldn't be bad to get acquainted with them.