How can we be so sure about the authenticity and genuineness of all the new "versions" and translations of the Canonical Bible?

What does "authenticity" and "genuineness" mean with regard to a translation? I'd rather use the term "accuracy," and you can only gauge accuracy of a translation by learning the original language and comparing the translation to it. Even without learning to read Hebrew and Greek fluently there are a slew of tools that can be used to help the individual in this task. Interlinears, Lexicons, etc. Many of them are freely available online.

As to your examples, quite simply the Greek text does say Jesus had brothers, and even most Catholic translations will translate the word adelphoi (Greek for brothers) as brothers. Only a biased translation would change it to "cousins" to save Catholic orthodoxy from the actual text of the Bible. Catholics generally just explain that "brother" can also mean cousin (because it can be used in a non-literal sense) rather than change the text, so I'm not sure what you're complaining about the Reina Valera's translation here for.

The debate over the meaning of alma in Isaiah 7:14 is a big one. But it doesn't affect the translation in Matthew. Matthew's quotation of Isaiah 7:14 will always say "virgin" in every translation. Where translations differ is in the Old Testament, in Isaiah 7:14 itself. This is because although the Septuagint (aka LXX, the Greek translation of the Old Testament) translated alma into Greek as parthenos (virgin), and although Matthew quotes it as such, the Jews complain about it. So to appease the Jews in their Old Testament translations some modern translations translate it in its Old Testament context the way that pleases the Jews (i.e. young woman) while retaining virgin in Matthew. Its political correctness and ecumenism, in other words.

Most of these issues are well known to people interested in verifying the correctness of the translations, and it just takes a little research on the internet to get familiar with these issues. I don't consider it a big deal, and I don't think we need for the versions to be "moderated."

That being said, since you are Catholic (apparently) you ought to be informed that the Catholic church only approves some translations for liturgical use (i.e. to be read at mass) and most Catholics seem to stick with those translations. Its not required that Catholics only use the approved translations for private study, but many Catholics decide to do so.

I will note on this point, however, that some of the translations approved by the hierarchy are just as flawed, or more flawed, than Protestant translations in the very areas you have an issue with. For example, saying "young woman" in Isaiah 7:14. The Catholic hierarchy approves of the RSV and NRSV in certain contexts, and both of them have this flaw, while many Protestant translations do not. RSV was the official U.S. translation prior to the NAB, and is still used by the Vatican in its English publications. The NRSV is the official Canadian translation used in Canadian missals. The NAB is the current official Catholic translation for the U.S., and is probably one of the worst translations of all time. Even most Catholics will admit this, many opting for NRSV Catholic Edition and others in their private reading instead because of how the NAB dumbs things down and is constantly under revision in a vain attempt to make it useable.

I said everything in the preceding paragraph because of the "moderation" question. The Vatican's track record of "moderating" modern English translations is not so good. When it comes to preventing translations from diluting Messianic passages in the Old Testament to appease vocal Jewish critics, the Vatican will probably be the worst "moderator" because of their stance that the Old Covenant is still salvific and Jews don't have to accept Christ, etc. and their ecumenical agendas. Protestants are more likely to maintain the traditional Christian translations of Messianic texts in the Old Testament because they tend to be more committed to the idea that apart from faith in Christ there can be no salvation. And amazingly Protestantism seems to be able to exercise this kind of oversight over translations without a central head. It is lay oversight, in a way. Large bodies of laymen don't rush out and buy your new translation if you change Isaiah 7:14 to say "young woman" rather than "virgin." Translations that pull these stunts, even if initially successful, will begin to lose market-share after laymen figure it out and spread by word of mouth that the translation is corrupt. That's what happened to the RSV. And once Protestants abandoned it en masse as a corrupt translation, the Vatican picked it up, resulting in it being impossible today to find a Protestant RSV, since only RSV-CE (RSV Catholic Editions) are even sold today.